diff --git a/src/Model/Questionnaire/Condition.php b/src/Model/Questionnaire/Condition.php
index 8fab3c3..1b64ea8 100644
--- a/src/Model/Questionnaire/Condition.php
+++ b/src/Model/Questionnaire/Condition.php
@@ -19,4 +19,26 @@ readonly class Condition
         public string|array|int $value,
     ) {
     }
+
+    /**
+     * Whether the controlling question's answer satisfies this condition. An unanswered question
+     * satisfies none: a follow-up stays hidden until there is something to follow up on.
+     *
+     * This says nothing about whether the controlling question is itself shown; that is
+     * Definition::isVisible().
+     *
+     * @param array<string, mixed> $answers keyed by question key
+     */
+    public function isMet(array $answers): bool
+    {
+        $answer = $answers[$this->question] ?? null;
+
+        return match ($this->operator) {
+            ConditionOperator::EQUALS => $answer === $this->value,
+            ConditionOperator::IN => in_array($answer, $this->value, true),
+            // A rating arrives as an int from the form and from stored JSON alike; anything else
+            // is not a rating.
+            ConditionOperator::LTE => is_int($answer) && $answer <= $this->value,
+        };
+    }
 }
diff --git a/src/Model/Questionnaire/Definition.php b/src/Model/Questionnaire/Definition.php
index 0ee3a50..0764cf9 100644
--- a/src/Model/Questionnaire/Definition.php
+++ b/src/Model/Questionnaire/Definition.php
@@ -40,4 +40,27 @@ readonly class Definition
     {
         return $this->getQuestions()[$key] ?? null;
     }
+
+    /**
+     * Whether a question is shown for these answers: it has no show_if, or its condition is met
+     * and the question it depends on is shown in turn. A hidden question's answer counts for
+     * nothing, even if one was given before it was hidden - otherwise a follow-up could stay open
+     * on an answer the respondent can no longer see.
+     *
+     * Terminates because a show_if may only name an earlier question (QuestionnaireCatalog).
+     *
+     * @param array<string, mixed> $answers keyed by question key
+     */
+    public function isVisible(Question $question, array $answers): bool
+    {
+        if (null === $question->showIf) {
+            return true;
+        }
+
+        $controlling = $this->getQuestion($question->showIf->question);
+
+        return null !== $controlling
+            && $this->isVisible($controlling, $answers)
+            && $question->showIf->isMet($answers);
+    }
 }
diff --git a/tests/Model/Questionnaire/ConditionTest.php b/tests/Model/Questionnaire/ConditionTest.php
new file mode 100644
index 0000000..427c407
--- /dev/null
+++ b/tests/Model/Questionnaire/ConditionTest.php
@@ -0,0 +1,55 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Tests\Model\Questionnaire;
+
+use App\Enum\ConditionOperator;
+use App\Model\Questionnaire\Condition;
+use PHPUnit\Framework\Attributes\DataProvider;
+use PHPUnit\Framework\TestCase;
+
+class ConditionTest extends TestCase
+{
+    #[DataProvider('provideAnswers')]
+    public function testIsMet(ConditionOperator $operator, string|array|int $value, mixed $answer, bool $expected): void
+    {
+        $condition = new Condition('controlling', $operator, $value);
+
+        $this->assertSame($expected, $condition->isMet(['controlling' => $answer]));
+    }
+
+    public static function provideAnswers(): iterable
+    {
+        yield 'equals, matching' => [ConditionOperator::EQUALS, 'yes', 'yes', true];
+        yield 'equals, other option' => [ConditionOperator::EQUALS, 'yes', 'no', false];
+        yield 'in, listed' => [ConditionOperator::IN, ['bad', 'very_bad'], 'bad', true];
+        yield 'in, not listed' => [ConditionOperator::IN, ['bad', 'very_bad'], 'good', false];
+        yield 'lte, below' => [ConditionOperator::LTE, 3, 1, true];
+        yield 'lte, at the limit' => [ConditionOperator::LTE, 3, 3, true];
+        yield 'lte, above' => [ConditionOperator::LTE, 3, 4, false];
+        // A string "2" is not what the form or the stored JSON hands over for a rating, so it
+        // must not open a follow-up by accident.
+        yield 'lte, not an int' => [ConditionOperator::LTE, 3, '2', false];
+    }
+
+    /**
+     * An unanswered rating is not "3 or less": the low-rating follow-up must not demand a reason
+     * before a rating was given.
+     */
+    #[DataProvider('provideOperators')]
+    public function testUnansweredIsNeverMet(ConditionOperator $operator, string|array|int $value): void
+    {
+        $condition = new Condition('controlling', $operator, $value);
+
+        $this->assertFalse($condition->isMet([]));
+        $this->assertFalse($condition->isMet(['controlling' => null]));
+    }
+
+    public static function provideOperators(): iterable
+    {
+        yield 'equals' => [ConditionOperator::EQUALS, 'yes'];
+        yield 'in' => [ConditionOperator::IN, ['yes']];
+        yield 'lte' => [ConditionOperator::LTE, 3];
+    }
+}
diff --git a/tests/Model/Questionnaire/DefinitionTest.php b/tests/Model/Questionnaire/DefinitionTest.php
new file mode 100644
index 0000000..cd1a068
--- /dev/null
+++ b/tests/Model/Questionnaire/DefinitionTest.php
@@ -0,0 +1,70 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Tests\Model\Questionnaire;
+
+use App\Enum\ConditionOperator;
+use App\Enum\QuestionType;
+use App\Model\Questionnaire\Condition;
+use App\Model\Questionnaire\Definition;
+use App\Model\Questionnaire\Option;
+use App\Model\Questionnaire\Question;
+use App\Model\Questionnaire\Section;
+use PHPUnit\Framework\TestCase;
+
+class DefinitionTest extends TestCase
+{
+    public function testQuestionWithoutConditionIsAlwaysVisible(): void
+    {
+        $definition = $this->definition();
+
+        $this->assertTrue($definition->isVisible($definition->getQuestion('damage'), []));
+    }
+
+    public function testFollowUpIsVisibleOnlyForItsAnswer(): void
+    {
+        $definition = $this->definition();
+        $followUp = $definition->getQuestion('claim');
+
+        $this->assertTrue($definition->isVisible($followUp, ['damage' => 'yes']));
+        $this->assertFalse($definition->isVisible($followUp, ['damage' => 'no']));
+    }
+
+    /**
+     * "details" depends on "claim", which depends on "damage". Switching "damage" to "no" hides
+     * "claim"; its stale answer must not keep "details" open.
+     */
+    public function testFollowUpOfAHiddenQuestionIsHidden(): void
+    {
+        $definition = $this->definition();
+        $details = $definition->getQuestion('details');
+
+        $this->assertTrue($definition->isVisible($details, ['damage' => 'yes', 'claim' => 'expected']));
+        $this->assertFalse($definition->isVisible($details, ['damage' => 'no', 'claim' => 'expected']));
+    }
+
+    private function definition(): Definition
+    {
+        $yesNo = [new Option('yes', 'Ja'), new Option('no', 'Nein')];
+
+        return new Definition('sample', 1, 'Beispiel', null, [
+            new Section('main', null, [
+                new Question('damage', 'Schäden?', QuestionType::CHOICE, options: $yesNo),
+                new Question(
+                    'claim',
+                    'Kommt noch was?',
+                    QuestionType::CHOICE,
+                    options: [new Option('expected', 'Ja'), new Option('settled', 'Nein')],
+                    showIf: new Condition('damage', ConditionOperator::EQUALS, 'yes'),
+                ),
+                new Question(
+                    'details',
+                    'Was ist passiert?',
+                    QuestionType::TEXTAREA,
+                    showIf: new Condition('claim', ConditionOperator::EQUALS, 'expected'),
+                ),
+            ]),
+        ]);
+    }
+}