diff --git a/src/Controller/UsedInController.php b/src/Controller/UsedInController.php
new file mode 100644
index 0000000..85c15e1
--- /dev/null
+++ b/src/Controller/UsedInController.php
@@ -0,0 +1,204 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Controller;
+
+use App\Entity\ComposantPieceSlot;
+use App\Entity\ComposantProductSlot;
+use App\Entity\MachineComponentLink;
+use App\Entity\MachinePieceLink;
+use App\Entity\MachineProductLink;
+use App\Entity\PieceProductSlot;
+use Doctrine\ORM\EntityManagerInterface;
+use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
+use Symfony\Component\HttpFoundation\JsonResponse;
+use Symfony\Component\Routing\Attribute\Route;
+
+final class UsedInController extends AbstractController
+{
+    public function __construct(
+        private readonly EntityManagerInterface $em,
+    ) {}
+
+    #[Route('/api/composants/{id}/used-in', name: 'api_composant_used_in', methods: ['GET'])]
+    public function composantUsedIn(string $id): JsonResponse
+    {
+        $this->denyAccessUnlessGranted('ROLE_VIEWER');
+
+        return new JsonResponse([
+            'machines' => $this->findMachinesUsingComposant($id),
+        ]);
+    }
+
+    #[Route('/api/pieces/{id}/used-in', name: 'api_piece_used_in', methods: ['GET'])]
+    public function pieceUsedIn(string $id): JsonResponse
+    {
+        $this->denyAccessUnlessGranted('ROLE_VIEWER');
+
+        return new JsonResponse([
+            'machines'   => $this->findMachinesUsingPiece($id),
+            'composants' => $this->findComposantsUsingPiece($id),
+        ]);
+    }
+
+    #[Route('/api/products/{id}/used-in', name: 'api_product_used_in', methods: ['GET'])]
+    public function productUsedIn(string $id): JsonResponse
+    {
+        $this->denyAccessUnlessGranted('ROLE_VIEWER');
+
+        return new JsonResponse([
+            'machines'   => $this->findMachinesUsingProduct($id),
+            'composants' => $this->findComposantsUsingProduct($id),
+            'pieces'     => $this->findPiecesUsingProduct($id),
+        ]);
+    }
+
+    /**
+     * @return list<array{id: string, name: null|string, site: null|array{id: string, name: null|string}}>
+     */
+    private function findMachinesUsingComposant(string $id): array
+    {
+        $qb = $this->em->createQueryBuilder();
+        $qb->select('m.id', 'm.name', 's.id as siteId', 's.name as siteName')
+            ->from(MachineComponentLink::class, 'mcl')
+            ->join('mcl.machine', 'm')
+            ->leftJoin('m.site', 's')
+            ->where('mcl.composant = :id')
+            ->setParameter('id', $id)
+            ->groupBy('m.id', 'm.name', 's.id', 's.name')
+            ->orderBy('m.name', 'ASC')
+        ;
+
+        return $this->formatMachineResults($qb->getQuery()->getArrayResult());
+    }
+
+    /**
+     * @return list<array{id: string, name: null|string, site: null|array{id: string, name: null|string}}>
+     */
+    private function findMachinesUsingPiece(string $id): array
+    {
+        $qb = $this->em->createQueryBuilder();
+        $qb->select('m.id', 'm.name', 's.id as siteId', 's.name as siteName')
+            ->from(MachinePieceLink::class, 'mpl')
+            ->join('mpl.machine', 'm')
+            ->leftJoin('m.site', 's')
+            ->where('mpl.piece = :id')
+            ->setParameter('id', $id)
+            ->groupBy('m.id', 'm.name', 's.id', 's.name')
+            ->orderBy('m.name', 'ASC')
+        ;
+
+        return $this->formatMachineResults($qb->getQuery()->getArrayResult());
+    }
+
+    /**
+     * @return list<array{id: string, name: null|string, site: null|array{id: string, name: null|string}}>
+     */
+    private function findMachinesUsingProduct(string $id): array
+    {
+        $qb = $this->em->createQueryBuilder();
+        $qb->select('m.id', 'm.name', 's.id as siteId', 's.name as siteName')
+            ->from(MachineProductLink::class, 'mpl')
+            ->join('mpl.machine', 'm')
+            ->leftJoin('m.site', 's')
+            ->where('mpl.product = :id')
+            ->setParameter('id', $id)
+            ->groupBy('m.id', 'm.name', 's.id', 's.name')
+            ->orderBy('m.name', 'ASC')
+        ;
+
+        return $this->formatMachineResults($qb->getQuery()->getArrayResult());
+    }
+
+    /**
+     * @return list<array{id: string, name: null|string}>
+     */
+    private function findComposantsUsingPiece(string $id): array
+    {
+        $qb = $this->em->createQueryBuilder();
+        $qb->select('c.id', 'c.name')
+            ->from(ComposantPieceSlot::class, 'cps')
+            ->join('cps.composant', 'c')
+            ->where('cps.selectedPiece = :id')
+            ->setParameter('id', $id)
+            ->groupBy('c.id', 'c.name')
+            ->orderBy('c.name', 'ASC')
+        ;
+
+        return array_values(array_map(
+            static fn (array $row): array => [
+                'id'   => $row['id'],
+                'name' => $row['name'],
+            ],
+            $qb->getQuery()->getArrayResult(),
+        ));
+    }
+
+    /**
+     * @return list<array{id: string, name: null|string}>
+     */
+    private function findComposantsUsingProduct(string $id): array
+    {
+        $qb = $this->em->createQueryBuilder();
+        $qb->select('c.id', 'c.name')
+            ->from(ComposantProductSlot::class, 'cps')
+            ->join('cps.composant', 'c')
+            ->where('cps.selectedProduct = :id')
+            ->setParameter('id', $id)
+            ->groupBy('c.id', 'c.name')
+            ->orderBy('c.name', 'ASC')
+        ;
+
+        return array_values(array_map(
+            static fn (array $row): array => [
+                'id'   => $row['id'],
+                'name' => $row['name'],
+            ],
+            $qb->getQuery()->getArrayResult(),
+        ));
+    }
+
+    /**
+     * @return list<array{id: string, name: null|string}>
+     */
+    private function findPiecesUsingProduct(string $id): array
+    {
+        $qb = $this->em->createQueryBuilder();
+        $qb->select('p.id', 'p.name')
+            ->from(PieceProductSlot::class, 'pps')
+            ->join('pps.piece', 'p')
+            ->where('pps.selectedProduct = :id')
+            ->setParameter('id', $id)
+            ->groupBy('p.id', 'p.name')
+            ->orderBy('p.name', 'ASC')
+        ;
+
+        return array_values(array_map(
+            static fn (array $row): array => [
+                'id'   => $row['id'],
+                'name' => $row['name'],
+            ],
+            $qb->getQuery()->getArrayResult(),
+        ));
+    }
+
+    /**
+     * @param array<int, array<string, mixed>> $rows
+     *
+     * @return list<array{id: string, name: null|string, site: null|array{id: string, name: null|string}}>
+     */
+    private function formatMachineResults(array $rows): array
+    {
+        return array_values(array_map(
+            static fn (array $row): array => [
+                'id'   => $row['id'],
+                'name' => $row['name'],
+                'site' => $row['siteId']
+                    ? ['id' => $row['siteId'], 'name' => $row['siteName']]
+                    : null,
+            ],
+            $rows,
+        ));
+    }
+}