diff --git a/.lintr b/.lintr
index fb125ea..3d4eb22 100644
--- a/.lintr
+++ b/.lintr
@@ -1,2 +1,2 @@
 linters: linters_with_defaults() # see vignette("lintr")
-encoding: "UTF-8"
\ No newline at end of file
+encoding: "UTF-8"
diff --git a/NAMESPACE b/NAMESPACE
index 98947ce..0785192 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -1,6 +1,16 @@
 # Generated by roxygen2: do not edit by hand
 
+export(uis_get)
+export(uis_get_entities)
+export(uis_get_versions)
+export(uis_get_versions_default)
 import(cli)
 import(httr2)
 import(jsonlite)
 import(tibble)
+importFrom(dplyr,filter)
+importFrom(dplyr,mutate)
+importFrom(dplyr,rename)
+importFrom(dplyr,select)
+importFrom(tidyr,nest)
+importFrom(tidyr,unnest)
diff --git a/R/perform_request.R b/R/perform_request.R
index ab855c1..c83da99 100644
--- a/R/perform_request.R
+++ b/R/perform_request.R
@@ -4,7 +4,8 @@ perform_request <- function(
   resource,
   entities = NULL,
   indicators = NULL,
-  time = NULL,
+  start_year = NULL,
+  end_year = NULL,
   version = NULL,
   disaggregations = NULL,
   glossary_terms = NULL
@@ -16,11 +17,14 @@ perform_request <- function(
   req <- request(base_url) |>
     req_url_path_append(resource) |>
     req_url_query(
-      entities = entities,
-
+      geoUnit = entities,
+      indicator = indicators,
+      start = start_year,
+      end = end_year,
       version = version,
       disaggregations = disaggregations,
-      glossary_terms = glossary_terms
+      glossary_terms = glossary_terms,
+      .multi = "explode"
     )
 
   tryCatch(
@@ -41,4 +45,6 @@ perform_request <- function(
       )
     }
   )
+
+  resp
 }
diff --git a/R/uis_get.R b/R/uis_get.R
index cebf565..7c38833 100644
--- a/R/uis_get.R
+++ b/R/uis_get.R
@@ -1,3 +1,50 @@
+#' Get data from the UIS API
+#'
+#' @description
+#' Retrieves data from the UNESCO Institute for Statistics (UIS) API for
+#' specified entities, indicators, and time periods.
+#'
+#' @param entities Character vector. The entity IDs (geoUnits) to retrieve data
+#'  for. Must provide either this parameter or `indicators` or both.
+#' @param indicators Character vector. The indicator IDs to retrieve data for.
+#'   Must provide either this parameter or `entities` or both.
+#' @param start_year Numeric or character. The starting year for the data
+#'  retrieval period. If NULL, no start year constraint is applied.
+#' @param end_year Numeric or character. The ending year for the data retrieval
+#'  period. If NULL, no end year constraint is applied.
+#' @param version Character. The API version to use. If NULL, the default
+#'  version is used.
+#'
+#' @return A data frame with the following columns:
+#'   \item{entity_id}{Character. The ID of the entity (geoUnit).}
+#'   \item{indicator_id}{Character. The ID of the indicator.}
+#'   \item{year}{Numeric. The year of the observation.}
+#'   \item{value}{Numeric. The value of the indicator for the given entity and
+#'  year.}
+#'
+#' @examplesIf curl::has_internet()
+#' \donttest{
+#' # Get a single indicator for all countries
+#' literacy_data <- uis_get(
+#'   indicators = "200101",
+#'   start_year = 2015,
+#'   end_year = 2020
+#' )
+#'
+#' # Get multiple indicators for a single country
+#' uis_get(
+#'   entities = "BRA",
+#'   indicators = c("GER.1", "NER.1", "CR.1"),
+#'   start_year = 2010,
+#'   end_year = 2020
+#' )
+#' }
+#'
+#' @seealso
+#' \link{uis_get_entities} for retrieving available geographical entities and
+#' \link{uis_get_versions} for retrieving available API versions
+#'
+#' @export
 uis_get <- function(
   entities = NULL,
   indicators = NULL,
@@ -5,24 +52,80 @@ uis_get <- function(
   end_year = NULL,
   version = NULL
 ) {
-  # TODO: at least entities or indicators must be given
+  if (is.null(entities) && is.null(indicators)) {
+    cli::cli_abort(
+      c(
+        "!" = "At least {.arg entities} or {.arg indicators} must be provided."
+      )
+    )
+  }
 
-  # TODO: add parameter validation
+  validate_character_vector(indicators, "indicators")
+  validate_character_vector(entities, "entities")
+  validate_year(start_year, "end_year")
+  validate_year(end_year, "end_year")
+  validate_version(version)
 
-  # TODO: add support for parameters
-  resp <- perform_request("data/indicators")
+  resp <- perform_request(
+    "data/indicators",
+    entities = entities,
+    indicators = indicators,
+    start_year = start_year,
+    end_year = end_year,
+    version = version
+  )
 
   data_raw <- resp |>
     resp_body_json(simplifyVector = TRUE)
 
-  data <- data_raw$records |>
-    as_tibble() |>
-    dplyr::select(
-      entity_id = geoUnit,
-      indicator_id = indicatorId,
-      year,
-      value
+  records <- data_raw$records
+
+  print(length(records))
+  if (length(records) == 0) {
+    cli::cli_inform(
+      c("i" = "No records found for this query.")
     )
+    return(invisible(NULL))
+  } else {
+    data <- records |>
+      as_tibble() |>
+      dplyr::select(
+        "entity_id" = "geoUnit",
+        "indicator_id" = "indicatorId",
+        "year",
+        "value"
+      )
+    return(data)
+  }
+}
+
+#' @noRd
+#' @keywords internal
+validate_character_vector <- function(arg, arg_name) {
+  if (!is.null(arg)) {
+    if (!is.character(arg) || any(is.na(arg))) {
+      cli::cli_abort(
+        c(
+          "!" = paste(
+            "{.arg {arg_name}} must be a character vector and cannot contain ",
+            "NA values."
+          )
+        )
+      )
+    }
+  }
+}
 
-  data
+#' @noRd
+#' @keywords internal
+validate_year <- function(year, arg_name) {
+  if (!is.null(year)) {
+    if (!is.numeric(year)) {
+      cli::cli_abort(
+        c(
+          "!" = paste("{.arg {arg_name}} must be an integer.")
+        )
+      )
+    }
+  }
 }
diff --git a/R/uis_get_entities.R b/R/uis_get_entities.R
index d777d7c..194be89 100644
--- a/R/uis_get_entities.R
+++ b/R/uis_get_entities.R
@@ -1,20 +1,51 @@
+#' Get geographical entities from UIS API
+#'
+#' @description
+#' Retrieves information about geographical entities (countries, regions, etc.)
+#' available in the UNESCO Institute for Statistics (UIS) API.
+#'
+#' @param version Character. The API version to use. If NULL (default), the
+#'  API's default version will be used.
+#'
+#' @return A data frame with information about geographical entities:
+#'   \item{entity_id}{Character. The unique identifier for the entity.}
+#'   \item{entity_name}{Character. The name of the geographical entity.}
+#'   \item{entity_typ}{Character. The type of entity (e.g., country, region).}
+#'   \item{region_group}{Character or list. Information about the region
+#' grouping.}
+#'
+#' @examples
+#' \donttest{
+#' uis_get_entities()
+#'
+#' # Download entities for a specific version
+#' uis_get_entities("20250225-2ae60fad")
+#' }
+#'
+#' @seealso
+#' \link{uis_get_versions} for retrieving available API versions and
+#' \link{uis_get_versions_default} for getting the default API version
+#'
+#' @export
 uis_get_entities <- function(
   version = NULL
 ) {
-  # TODO: add parameter validation
+  validate_version(version)
 
-  # TODO: add support for parameters
-  resp <- perform_request("definitions/geounits")
+  resp <- perform_request(
+    "definitions/geounits",
+    version = version
+  )
 
   entities_raw <- resp_body_json(resp) |>
     dplyr::bind_rows()
 
   entities <- entities_raw |>
     dplyr::rename(
-      entity_id = id,
-      entity_name = name,
-      entity_typ = type,
-      region_group = regionGroup
+      "entity_id" = "id",
+      "entity_name" = "name",
+      "entity_typ" = "type",
+      "region_group" = "regionGroup"
     )
 
   entities <- convert_to_snake_case(entities)
diff --git a/R/uis_get_indicators.R b/R/uis_get_indicators.R
index e4e8000..30c663a 100644
--- a/R/uis_get_indicators.R
+++ b/R/uis_get_indicators.R
@@ -3,20 +3,23 @@ uis_get_indicators <- function(
   disaggregations = NULL,
   glossary_terms = NULL
 ) {
-  # TODO: add parameter validation
+  validate_version(version)
 
   # TODO: add support for parameters
-  resp <- perform_request("definitions/indicators")
+  resp <- perform_request(
+    "definitions/indicators",
+    version = version
+  )
 
   indicators_raw <- resp_body_json(resp, simplifyVector = TRUE) |>
     as_tibble()
 
   # TODO: rename columns and nest them again afterwards with new names
   indicators <- indicators_raw |>
-    tidyr::unnest(dataAvailability) |>
-    tidyr::unnest(timeLine) |>
-    tidyr::unnest(geoUnits) |>
-    tidyr::unnest(types)
+    tidyr::unnest("dataAvailability") |>
+    tidyr::unnest("timeLine") |>
+    tidyr::unnest("geoUnits") |>
+    tidyr::unnest("types")
 
   indicators <- convert_to_snake_case(indicators)
 
diff --git a/R/uis_get_versions.R b/R/uis_get_versions.R
index dceb822..5698307 100644
--- a/R/uis_get_versions.R
+++ b/R/uis_get_versions.R
@@ -1,3 +1,31 @@
+#' Get available UIS API versions
+#'
+#' @description
+#' Retrieves information about the available versions of the UNESCO Institute
+#' for Statistics (UIS) API, including version IDs, descriptions, and theme
+#' data.
+#'
+#' @return A data frame with the following columns:
+#'   \item{id}{Character. The version identifier.}
+#'   \item{code}{Character. The version code.}
+#'   \item{description}{Character. The description of the version.}
+#'   \item{release_date}{Character. The date when the version was released.}
+#'   \item{theme}{List column. Each element is a nested data frame containing
+#' information about themes available in the version, with columns:
+#'     \itemize{
+#'       \item{theme_id}{Character. The theme identifier.}
+#'       \item{theme_last_update}{Character. The date when the theme was last
+#' updated.}
+#'       \item{theme_description}{Character. The description of the theme.}
+#'     }
+#'   }
+#'
+#' @examplesIf curl::has_internet()
+#' \donttest{
+#' uis_get_versions()
+#' }
+#'
+#' @export
 uis_get_versions <- function() {
   resp <- perform_request("versions")
 
@@ -5,17 +33,72 @@ uis_get_versions <- function() {
     as_tibble()
 
   versions <- versions_raw |>
-    dplyr::rename(versionDescription = description) |>
-    tidyr::unnest(themeDataStatus) |>
+    dplyr::rename("versionDescription" = "description") |>
+    tidyr::unnest("themeDataStatus") |>
     dplyr::rename(
-      themeId = theme,
-      themeLastUpdate = lastUpdate,
-      themeDescription = description
+      "themeId" = "theme",
+      "themeLastUpdate" = "lastUpdate",
+      "themeDescription" = "description"
     ) |>
-    dplyr::rename(description = versionDescription)
+    dplyr::rename(
+      "description" = "versionDescription"
+    )
 
   versions <- convert_to_snake_case(versions) |>
-    tidyr::nest(theme = c(theme_id, theme_last_update, theme_description))
+    tidyr::nest(
+      theme = c("theme_id", "theme_last_update", "theme_description")
+    )
 
   versions
 }
+
+#' @rdname uis_get_versions
+#' @export
+uis_get_versions_default <- function() {
+  resp <- perform_request("versions/default")
+
+  versions_default_raw <- resp_body_json(resp, simplifyVector = TRUE) |>
+    as_tibble()
+
+  versions_default <- versions_default_raw |>
+    dplyr::rename("versionDescription" = "description") |>
+    tidyr::unnest("themeDataStatus") |>
+    dplyr::rename(
+      "themeId" = "theme",
+      "themeLastUpdate" = "lastUpdate",
+      "themeDescription" = "description"
+    ) |>
+    dplyr::rename(
+      "description" = "versionDescription"
+    )
+
+  versions_default <- convert_to_snake_case(versions_default) |>
+    tidyr::nest(
+      theme = c("theme_id", "theme_last_update", "theme_description")
+    )
+
+  versions_default
+}
+
+#' @keywords internal
+#' @noRd
+validate_version <- function(version) {
+  if (!is.null(version)) {
+    if (!is.character(version)) {
+      cli::cli_abort(
+        c(
+          "!" = "{.arg version} must be a character."
+        )
+      )
+    }
+    version_length <- nchar(version)
+    if (version_length != 17) {
+      cli::cli_abort(
+        c(
+          "!" = "{.arg version} must have length 17.",
+          "x" = "You've supplied {version_length}."
+        )
+      )
+    }
+  }
+}
diff --git a/R/uis_get_versions_default.R b/R/uis_get_versions_default.R
deleted file mode 100644
index ce8277b..0000000
--- a/R/uis_get_versions_default.R
+++ /dev/null
@@ -1,21 +0,0 @@
-uis_get_versions_default <- function() {
-  resp <- perform_request("versions/default")
-
-  versions_default_raw <- resp_body_json(resp, simplifyVector = TRUE) |>
-    as_tibble()
-
-  versions_default <- versions_default_raw |>
-    dplyr::rename(versionDescription = description) |>
-    tidyr::unnest(themeDataStatus) |>
-    dplyr::rename(
-      themeId = theme,
-      themeLastUpdate = lastUpdate,
-      themeDescription = description
-    ) |>
-    dplyr::rename(description = versionDescription)
-
-  versions_default <- convert_to_snake_case(versions_default) |>
-    tidyr::nest(theme = c(theme_id, theme_last_update, theme_description))
-
-  versions_default
-}
diff --git a/man/uis_get.Rd b/man/uis_get.Rd
new file mode 100644
index 0000000..d3afe4e
--- /dev/null
+++ b/man/uis_get.Rd
@@ -0,0 +1,66 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/uis_get.R
+\name{uis_get}
+\alias{uis_get}
+\title{Get data from the UIS API}
+\usage{
+uis_get(
+  entities = NULL,
+  indicators = NULL,
+  start_year = NULL,
+  end_year = NULL,
+  version = NULL
+)
+}
+\arguments{
+\item{entities}{Character vector. The entity IDs (geoUnits) to retrieve data
+for. Must provide either this parameter or \code{indicators} or both.}
+
+\item{indicators}{Character vector. The indicator IDs to retrieve data for.
+Must provide either this parameter or \code{entities} or both.}
+
+\item{start_year}{Numeric or character. The starting year for the data
+retrieval period. If NULL, no start year constraint is applied.}
+
+\item{end_year}{Numeric or character. The ending year for the data retrieval
+period. If NULL, no end year constraint is applied.}
+
+\item{version}{Character. The API version to use. If NULL, the default
+version is used.}
+}
+\value{
+A data frame with the following columns:
+\item{entity_id}{Character. The ID of the entity (geoUnit).}
+\item{indicator_id}{Character. The ID of the indicator.}
+\item{year}{Numeric. The year of the observation.}
+\item{value}{Numeric. The value of the indicator for the given entity and
+year.}
+}
+\description{
+Retrieves data from the UNESCO Institute for Statistics (UIS) API for
+specified entities, indicators, and time periods.
+}
+\examples{
+\dontshow{if (curl::has_internet()) (if (getRversion() >= "3.4") withAutoprint else force)(\{ # examplesIf}
+\donttest{
+# Get a single indicator for all countries
+literacy_data <- uis_get(
+  indicators = "200101",
+  start_year = 2015,
+  end_year = 2020
+)
+
+# Get multiple indicators for a single country
+uis_get(
+  entities = "BRA",
+  indicators = c("GER.1", "NER.1", "CR.1"),
+  start_year = 2010,
+  end_year = 2020
+)
+}
+\dontshow{\}) # examplesIf}
+}
+\seealso{
+\link{uis_get_entities} for retrieving available geographical entities and
+\link{uis_get_versions} for retrieving available API versions
+}
diff --git a/man/uis_get_entities.Rd b/man/uis_get_entities.Rd
new file mode 100644
index 0000000..98973a2
--- /dev/null
+++ b/man/uis_get_entities.Rd
@@ -0,0 +1,37 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/uis_get_entities.R
+\name{uis_get_entities}
+\alias{uis_get_entities}
+\title{Get geographical entities from UIS API}
+\usage{
+uis_get_entities(version = NULL)
+}
+\arguments{
+\item{version}{Character. The API version to use. If NULL (default), the
+API's default version will be used.}
+}
+\value{
+A data frame with information about geographical entities:
+\item{entity_id}{Character. The unique identifier for the entity.}
+\item{entity_name}{Character. The name of the geographical entity.}
+\item{entity_typ}{Character. The type of entity (e.g., country, region).}
+\item{region_group}{Character or list. Information about the region
+grouping.}
+}
+\description{
+Retrieves information about geographical entities (countries, regions, etc.)
+available in the UNESCO Institute for Statistics (UIS) API.
+}
+\examples{
+\donttest{
+uis_get_entities()
+
+# Download entities for a specific version
+uis_get_entities("20250225-2ae60fad")
+}
+
+}
+\seealso{
+\link{uis_get_versions} for retrieving available API versions and
+\link{uis_get_versions_default} for getting the default API version
+}
diff --git a/man/uis_get_versions.Rd b/man/uis_get_versions.Rd
new file mode 100644
index 0000000..209695c
--- /dev/null
+++ b/man/uis_get_versions.Rd
@@ -0,0 +1,39 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/uis_get_versions.R
+\name{uis_get_versions}
+\alias{uis_get_versions}
+\alias{uis_get_versions_default}
+\title{Get available UIS API versions}
+\usage{
+uis_get_versions()
+
+uis_get_versions_default()
+}
+\value{
+A data frame with the following columns:
+\item{id}{Character. The version identifier.}
+\item{code}{Character. The version code.}
+\item{description}{Character. The description of the version.}
+\item{release_date}{Character. The date when the version was released.}
+\item{theme}{List column. Each element is a nested data frame containing
+information about themes available in the version, with columns:
+\itemize{
+\item{theme_id}{Character. The theme identifier.}
+\item{theme_last_update}{Character. The date when the theme was last
+updated.}
+\item{theme_description}{Character. The description of the theme.}
+}
+}
+}
+\description{
+Retrieves information about the available versions of the UNESCO Institute
+for Statistics (UIS) API, including version IDs, descriptions, and theme
+data.
+}
+\examples{
+\dontshow{if (curl::has_internet()) (if (getRversion() >= "3.4") withAutoprint else force)(\{ # examplesIf}
+\donttest{
+uis_get_versions()
+}
+\dontshow{\}) # examplesIf}
+}
diff --git a/man/uisapi-package.Rd b/man/uisapi-package.Rd
index d830000..6fc7926 100644
--- a/man/uisapi-package.Rd
+++ b/man/uisapi-package.Rd
@@ -18,7 +18,7 @@ Useful links:
 
 }
 \author{
-\strong{Maintainer}: Christoph Scheuch \email{christophh@tidy-intelligence.com} (\href{https://orcid.org/0009-0004-0423-6819}{ORCID}) [copyright holder]
+\strong{Maintainer}: Christoph Scheuch \email{christoph@tidy-intelligence.com} (\href{https://orcid.org/0009-0004-0423-6819}{ORCID}) [copyright holder]
 
 }
 \keyword{internal}