diff --git a/api/v1alpha1/microvmmachine_webhook.go b/api/v1alpha1/microvmmachine_webhook.go
index 468dd02..2e4a862 100644
--- a/api/v1alpha1/microvmmachine_webhook.go
+++ b/api/v1alpha1/microvmmachine_webhook.go
@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"reflect"
 
+	apierrors "k8s.io/apimachinery/pkg/api/errors"
 	"k8s.io/apimachinery/pkg/runtime"
 	"k8s.io/apimachinery/pkg/util/validation/field"
 	ctrl "sigs.k8s.io/controller-runtime"
@@ -34,7 +35,15 @@ var (
 
 // ValidateCreate implements webhook.Validator so a webhook will be registered for the type.
 func (r *MicrovmMachine) ValidateCreate() error {
-	return nil
+	var allErrs field.ErrorList
+
+	allErrs = append(allErrs, r.Spec.MicrovmSpec.Validate()...)
+
+	if len(allErrs) == 0 {
+		return nil
+	}
+
+	return apierrors.NewInvalid(r.GroupVersionKind().GroupKind(), r.Name, allErrs)
 }
 
 // ValidateDelete implements webhook.Validator so a webhook will be registered for the type.
diff --git a/api/v1alpha1/types.go b/api/v1alpha1/types.go
index 4fd1e15..44f3c4e 100644
--- a/api/v1alpha1/types.go
+++ b/api/v1alpha1/types.go
@@ -20,7 +20,7 @@ type MicrovmSpec struct {
 	MemoryMb int64 `json:"memoryMb"`
 
 	// RootVolume specifies the volume to use for the root of the microvm.
-	// +kubebuilder:validation:Required
+	// +optional
 	RootVolume Volume `json:"rootVolume"`
 
 	// AdditionalVolumes specifies additional non-root volumes to attach to the microvm.
@@ -28,7 +28,7 @@ type MicrovmSpec struct {
 	AdditionalVolumes []Volume `json:"volumes,omitempty"`
 
 	// Kernel specifies the kernel and its arguments to use.
-	// +kubebuilder:validation:Required
+	// +optional
 	Kernel ContainerFileSource `json:"kernel"`
 
 	// KernelCmdLine are the additional args to use for the kernel cmdline.
diff --git a/api/v1alpha1/validate.go b/api/v1alpha1/validate.go
index 2edcd36..83a272b 100644
--- a/api/v1alpha1/validate.go
+++ b/api/v1alpha1/validate.go
@@ -18,3 +18,33 @@ func (p *Placement) Validate() []*field.Error {
 
 	return errs
 }
+
+func (s *MicrovmSpec) Validate() []*field.Error {
+	var errs field.ErrorList
+
+	if s.OsVersion == "" {
+		if s.RootVolume.ID == "" {
+			errs = append(errs,
+				field.Required(field.NewPath("spec", "rootVolume", "id"), "must be set if osVersion is omitted"))
+		}
+
+		if s.RootVolume.Image == "" {
+			errs = append(errs,
+				field.Required(field.NewPath("spec", "rootVolume", "image"), "must be set if osVersion is omitted"))
+		}
+	}
+
+	if s.KernelVersion == "" {
+		if s.Kernel.Image == "" {
+			errs = append(errs,
+				field.Required(field.NewPath("spec", "kernel", "image"), "must be set if kernelVersion is omitted"))
+		}
+
+		if s.Kernel.Filename == "" {
+			errs = append(errs,
+				field.Required(field.NewPath("spec", "kernel", "filename"), "must be set if kernelVersion is omitted"))
+		}
+	}
+
+	return errs
+}
diff --git a/config/crd/bases/infrastructure.cluster.x-k8s.io_microvmmachines.yaml b/config/crd/bases/infrastructure.cluster.x-k8s.io_microvmmachines.yaml
index fa96743..5d15804 100644
--- a/config/crd/bases/infrastructure.cluster.x-k8s.io_microvmmachines.yaml
+++ b/config/crd/bases/infrastructure.cluster.x-k8s.io_microvmmachines.yaml
@@ -198,10 +198,8 @@ spec:
                   type: object
                 type: array
             required:
-            - kernel
             - memoryMb
             - networkInterfaces
-            - rootVolume
             - vcpu
             type: object
           status:
diff --git a/config/crd/bases/infrastructure.cluster.x-k8s.io_microvmmachinetemplates.yaml b/config/crd/bases/infrastructure.cluster.x-k8s.io_microvmmachinetemplates.yaml
index 8712ace..3aef3c3 100644
--- a/config/crd/bases/infrastructure.cluster.x-k8s.io_microvmmachinetemplates.yaml
+++ b/config/crd/bases/infrastructure.cluster.x-k8s.io_microvmmachinetemplates.yaml
@@ -241,10 +241,8 @@ spec:
                           type: object
                         type: array
                     required:
-                    - kernel
                     - memoryMb
                     - networkInterfaces
-                    - rootVolume
                     - vcpu
                     type: object
                 required: