diff --git a/.claude/settings.local.json b/.claude/settings.local.json
index ccda790..41dc01a 100644
--- a/.claude/settings.local.json
+++ b/.claude/settings.local.json
@@ -29,7 +29,8 @@
       "mcp__playwright__browser_wait_for",
       "WebFetch(domain:www.bakertilly.nl)",
       "mcp__playwright__browser_type",
-      "mcp__playwright__browser_hover"
+      "mcp__playwright__browser_hover",
+      "mcp__playwright__browser_evaluate"
     ]
   }
 }
diff --git a/app/Http/Controllers/SessionController.php b/app/Http/Controllers/SessionController.php
index 1d15ec0..06d984c 100644
--- a/app/Http/Controllers/SessionController.php
+++ b/app/Http/Controllers/SessionController.php
@@ -11,6 +11,7 @@
 use Illuminate\Http\RedirectResponse;
 use Illuminate\Http\Request;
 use Illuminate\Support\Arr;
+use Illuminate\Validation\ValidationException;
 use Inertia\Inertia;
 use Inertia\Response;
 
@@ -109,6 +110,8 @@ private function saveAnswers(Session $session, array $answers): void
      */
     private function completeSession(Session $session): RedirectResponse
     {
+        $this->validateSessionCompletion($session);
+
         $scoringService = new ScoringService;
         $score = $scoringService->calculateScore($session);
         $result = $scoringService->determineResult($score);
@@ -125,6 +128,76 @@ private function completeSession(Session $session): RedirectResponse
         return redirect()->route('sessions.result', $session);
     }
 
+    /**
+     * Validate that all required fields are answered before session completion.
+     */
+    private function validateSessionCompletion(Session $session): void
+    {
+        $session->load(['category.questionGroups.questions', 'answers']);
+
+        $errors = [];
+
+        foreach ($session->category->questionGroups as $questionGroup) {
+            foreach ($questionGroup->questions as $question) {
+                $answer = $session->answers->firstWhere('question_id', $question->id);
+
+                $this->validateRadioAnswer($question, $answer, $errors);
+                $this->validateDetailsAnswer($question, $answer, $errors);
+            }
+        }
+
+        if (Arr::exists($errors, 0)) {
+            throw ValidationException::withMessages([
+                'complete' => $errors,
+            ]);
+        }
+    }
+
+    /**
+     * Validate that radio button questions have an answer selected.
+     */
+    private function validateRadioAnswer($question, $answer, array &$errors): void
+    {
+        $hasRadioButtons = $question->has_yes || $question->has_no || $question->has_na;
+
+        if ($hasRadioButtons && (! $answer || $answer->value === null)) {
+            $errors[] = "Question '{$question->text}' requires an answer.";
+        }
+    }
+
+    /**
+     * Validate that questions with required details have text values provided.
+     */
+    private function validateDetailsAnswer($question, $answer, array &$errors): void
+    {
+        $details = $question->details;
+        $hasRadioButtons = $question->has_yes || $question->has_no || $question->has_na;
+
+        if ($details === 'required') {
+            if (! $answer || empty(trim(Arr::get($answer->toArray(), 'text_value', '')))) {
+                $errors[] = "Question '{$question->text}' requires details to be provided.";
+            }
+        }
+
+        if ($details === 'req_on_yes' && $answer && $answer->value === 'yes') {
+            if (empty(trim(Arr::get($answer->toArray(), 'text_value', '')))) {
+                $errors[] = "Question '{$question->text}' requires details when answered 'Yes'.";
+            }
+        }
+
+        if ($details === 'req_on_no' && $answer && $answer->value === 'no') {
+            if (empty(trim(Arr::get($answer->toArray(), 'text_value', '')))) {
+                $errors[] = "Question '{$question->text}' requires details when answered 'No'.";
+            }
+        }
+
+        if (! $hasRadioButtons && $details !== null && $details !== '') {
+            if (! $answer || empty(trim(Arr::get($answer->toArray(), 'text_value', '')))) {
+                $errors[] = "Question '{$question->text}' requires a text response.";
+            }
+        }
+    }
+
     /**
      * Display the final session result.
      */
diff --git a/resources/js/Components/QuestionCard.vue b/resources/js/Components/QuestionCard.vue
index a27a889..0e412fb 100644
--- a/resources/js/Components/QuestionCard.vue
+++ b/resources/js/Components/QuestionCard.vue
@@ -11,6 +11,10 @@ const props = defineProps({
     type: Object,
     default: () => ({ value: null, text_value: '' }),
   },
+  error: {
+    type: String,
+    default: null,
+  },
 })
 
 const emit = defineEmits(['update:modelValue'])
@@ -58,7 +62,10 @@ const updateTextValue = (event) => {
 </script>
 
 <template>
-    <div class="py-5 first:pt-0">
+    <div
+        class="py-5 first:pt-0 transition-all duration-200"
+        :class="{ 'border-l-2 border-red-400/60 pl-4 -ml-4': error }"
+    >
         <p class="text-white font-medium leading-relaxed mb-4">{{ question.text }}</p>
 
         <!-- Text-only question (no radio buttons) -->
@@ -105,5 +112,19 @@ const updateTextValue = (event) => {
                 </div>
             </Transition>
         </div>
+
+        <!-- Error message -->
+        <Transition
+            enter-active-class="transition-all duration-200 ease-out"
+            enter-from-class="opacity-0 -translate-y-1"
+            enter-to-class="opacity-100 translate-y-0"
+            leave-active-class="transition-all duration-150 ease-in"
+            leave-from-class="opacity-100 translate-y-0"
+            leave-to-class="opacity-0 -translate-y-1"
+        >
+            <p v-if="error" class="text-red-400 text-sm mt-2 bg-red-500/10 px-3 py-2 rounded-md">
+                {{ error }}
+            </p>
+        </Transition>
     </div>
 </template>
diff --git a/resources/js/Pages/Session/Show.vue b/resources/js/Pages/Session/Show.vue
index 49bb786..bf97df5 100644
--- a/resources/js/Pages/Session/Show.vue
+++ b/resources/js/Pages/Session/Show.vue
@@ -1,5 +1,5 @@
 <script setup>
-import { computed, reactive } from 'vue'
+import { computed, reactive, ref, watch, nextTick } from 'vue'
 import { Head, useForm, router } from '@inertiajs/vue3'
 import AppLayout from '@/Layouts/AppLayout.vue'
 import AppButton from '@/Components/AppButton.vue'
@@ -44,6 +44,11 @@ const initializeAnswers = () => {
 }
 initializeAnswers()
 
+// Validation state
+const validationErrors = ref({})
+const showErrors = ref(false)
+const questionRefs = ref({})
+
 // Save a single answer with partial reload including score
 let saveTimeout = null
 const saveAnswer = (questionId) => {
@@ -82,9 +87,79 @@ const saveComments = () => {
   }, 1000)
 }
 
-// Session completion
+// Validation function
+const validate = () => {
+  const errors = {}
+
+  props.questionGroups.forEach(group => {
+    group.questions.forEach(question => {
+      const answer = answerData[question.id]
+      const hasRadioButtons = question.has_yes || question.has_no || question.has_na
+
+      // Rule 1: Radio button questions must have a selection
+      if (hasRadioButtons && answer.value === null) {
+        errors[question.id] = 'Please select an answer'
+        return
+      }
+
+      // Rule 2: Required text fields based on details
+      if (question.details === 'required' && !answer.text_value?.trim()) {
+        errors[question.id] = 'Please provide details'
+        return
+      }
+
+      if (question.details === 'req_on_yes' && answer.value === 'yes' && !answer.text_value?.trim()) {
+        errors[question.id] = 'Please provide details'
+        return
+      }
+
+      if (question.details === 'req_on_no' && answer.value === 'no' && !answer.text_value?.trim()) {
+        errors[question.id] = 'Please provide details'
+        return
+      }
+
+      // Rule 3: Text-only questions (no radio buttons, has details)
+      if (!hasRadioButtons && question.details && !answer.text_value?.trim()) {
+        errors[question.id] = 'Please enter a response'
+        return
+      }
+    })
+  })
+
+  validationErrors.value = errors
+  return Object.keys(errors).length === 0
+}
+
+// Watch answerData for changes and revalidate when errors are showing
+watch(answerData, () => {
+  if (showErrors.value) {
+    validate()
+  }
+}, { deep: true })
+
+// Error count for summary banner
+const errorCount = computed(() => {
+  return Object.values(validationErrors.value).filter(err => err !== null).length
+})
+
+// Session completion with validation
 let completing = false
-const completeSession = () => {
+const completeSession = async () => {
+  showErrors.value = true
+
+  if (!validate()) {
+    // Scroll to first error
+    await nextTick()
+    const firstErrorQuestionId = Object.keys(validationErrors.value)[0]
+    const firstErrorElement = questionRefs.value[firstErrorQuestionId]
+
+    if (firstErrorElement) {
+      firstErrorElement.scrollIntoView({ behavior: 'smooth', block: 'center' })
+    }
+
+    return
+  }
+
   completing = true
   router.put(`/sessions/${props.session.id}`, {
     complete: true,
@@ -141,13 +216,19 @@ const hasScoredAnswers = computed(() => {
                 <p v-if="group.scoring_instructions" class="text-amber-400 text-sm italic mb-4">{{ group.scoring_instructions }}</p>
 
                 <div class="divide-y divide-white/[0.06]">
-                    <QuestionCard
+                    <div
                         v-for="question in group.questions"
                         :key="question.id"
-                        :question="question"
-                        :modelValue="answerData[question.id]"
-                        @update:modelValue="updateAnswer(question.id, $event)"
-                    />
+                        :ref="el => { if (el) questionRefs[question.id] = el }"
+                        :data-question-id="question.id"
+                    >
+                        <QuestionCard
+                            :question="question"
+                            :modelValue="answerData[question.id]"
+                            :error="showErrors ? validationErrors[question.id] : null"
+                            @update:modelValue="updateAnswer(question.id, $event)"
+                        />
+                    </div>
                 </div>
             </div>
 
@@ -164,8 +245,24 @@ const hasScoredAnswers = computed(() => {
             </div>
         </div>
 
-        <!-- Complete button - now enabled -->
+        <!-- Complete button with validation summary -->
         <div class="mt-12 pt-8 border-t border-white/[0.06]">
+            <!-- Validation summary banner -->
+            <Transition
+                enter-active-class="transition-all duration-200 ease-out"
+                enter-from-class="opacity-0 -translate-y-2"
+                enter-to-class="opacity-100 translate-y-0"
+                leave-active-class="transition-all duration-150 ease-in"
+                leave-from-class="opacity-100 translate-y-0"
+                leave-to-class="opacity-0 -translate-y-2"
+            >
+                <div v-if="showErrors && errorCount > 0" class="bg-red-500/10 border border-red-400/20 rounded-lg px-5 py-4 mb-6">
+                    <p class="text-red-400 text-sm font-medium">
+                        Please complete all required fields before submitting. {{ errorCount }} {{ errorCount === 1 ? 'question requires' : 'questions require' }} your attention.
+                    </p>
+                </div>
+            </Transition>
+
             <div class="flex justify-end">
                 <AppButton size="lg" @click="completeSession" data-cy="complete-session">
                     Complete
diff --git a/validation-banner.png b/validation-banner.png
new file mode 100644
index 0000000..c37135b
Binary files /dev/null and b/validation-banner.png differ
diff --git a/validation-summary.png b/validation-summary.png
new file mode 100644
index 0000000..1c7b2db
Binary files /dev/null and b/validation-summary.png differ