diff --git a/tests/unit/test_trainer.py b/tests/unit/test_trainer.py
new file mode 100644
index 0000000..aad7dd7
--- /dev/null
+++ b/tests/unit/test_trainer.py
@@ -0,0 +1,55 @@
+"""Tests for pet ID model trainer."""
+
+import os
+from pathlib import Path
+
+import numpy as np
+import pytest
+
+from vigilar.detection.trainer import PetTrainer, TrainingStatus
+
+
+class TestTrainingStatus:
+    def test_initial_status(self):
+        status = TrainingStatus()
+        assert status.is_training is False
+        assert status.progress == 0.0
+        assert status.error is None
+
+
+class TestPetTrainer:
+    def test_check_readiness_no_pets(self, tmp_path):
+        trainer = PetTrainer(training_dir=str(tmp_path), model_output_path=str(tmp_path / "model.pt"))
+        ready, msg = trainer.check_readiness(min_images=20)
+        assert not ready
+        assert "No pet" in msg
+
+    def test_check_readiness_insufficient_images(self, tmp_path):
+        pet_dir = tmp_path / "angel"
+        pet_dir.mkdir()
+        for i in range(5):
+            (pet_dir / f"{i}.jpg").write_bytes(b"fake")
+
+        trainer = PetTrainer(training_dir=str(tmp_path), model_output_path=str(tmp_path / "model.pt"))
+        ready, msg = trainer.check_readiness(min_images=20)
+        assert not ready
+        assert "angel" in msg.lower()
+
+    def test_check_readiness_sufficient_images(self, tmp_path):
+        for name in ["angel", "taquito"]:
+            pet_dir = tmp_path / name
+            pet_dir.mkdir()
+            for i in range(25):
+                (pet_dir / f"{i}.jpg").write_bytes(b"fake")
+
+        trainer = PetTrainer(training_dir=str(tmp_path), model_output_path=str(tmp_path / "model.pt"))
+        ready, msg = trainer.check_readiness(min_images=20)
+        assert ready
+
+    def test_get_class_names(self, tmp_path):
+        for name in ["angel", "milo", "taquito"]:
+            (tmp_path / name).mkdir()
+
+        trainer = PetTrainer(training_dir=str(tmp_path), model_output_path=str(tmp_path / "model.pt"))
+        names = trainer.get_class_names()
+        assert names == ["angel", "milo", "taquito"]  # sorted
diff --git a/vigilar/detection/trainer.py b/vigilar/detection/trainer.py
new file mode 100644
index 0000000..5a68fcc
--- /dev/null
+++ b/vigilar/detection/trainer.py
@@ -0,0 +1,141 @@
+"""Pet ID model trainer using MobileNetV3-Small with transfer learning."""
+
+import logging
+import shutil
+from dataclasses import dataclass, field
+from pathlib import Path
+
+log = logging.getLogger(__name__)
+
+
+@dataclass
+class TrainingStatus:
+    is_training: bool = False
+    progress: float = 0.0
+    epoch: int = 0
+    total_epochs: int = 0
+    accuracy: float = 0.0
+    error: str | None = None
+
+
+class PetTrainer:
+    def __init__(self, training_dir: str, model_output_path: str):
+        self._training_dir = Path(training_dir)
+        self._model_output_path = Path(model_output_path)
+        self.status = TrainingStatus()
+
+    def get_class_names(self) -> list[str]:
+        if not self._training_dir.exists():
+            return []
+        return sorted([
+            d.name for d in self._training_dir.iterdir()
+            if d.is_dir() and not d.name.startswith(".")
+        ])
+
+    def check_readiness(self, min_images: int = 20) -> tuple[bool, str]:
+        class_names = self.get_class_names()
+        if not class_names:
+            return False, "No pet directories found in training directory."
+
+        insufficient = []
+        for name in class_names:
+            pet_dir = self._training_dir / name
+            image_count = sum(1 for f in pet_dir.iterdir()
+                              if f.suffix.lower() in (".jpg", ".jpeg", ".png"))
+            if image_count < min_images:
+                insufficient.append(f"{name}: {image_count}/{min_images}")
+
+        if insufficient:
+            return False, f"Insufficient training images: {', '.join(insufficient)}"
+
+        return True, f"Ready to train with {len(class_names)} classes."
+
+    def train(self, epochs: int = 30, batch_size: int = 16) -> bool:
+        try:
+            import torch
+            import torch.nn as nn
+            from torch.utils.data import DataLoader
+            from torchvision import datasets, models, transforms
+
+            self.status = TrainingStatus(is_training=True, total_epochs=epochs)
+            class_names = self.get_class_names()
+            num_classes = len(class_names)
+
+            if num_classes < 2:
+                self.status.error = "Need at least 2 pets to train."
+                self.status.is_training = False
+                return False
+
+            train_transform = transforms.Compose([
+                transforms.Resize((256, 256)),
+                transforms.RandomCrop(224),
+                transforms.RandomHorizontalFlip(),
+                transforms.ColorJitter(brightness=0.2, contrast=0.2, saturation=0.2),
+                transforms.ToTensor(),
+                transforms.Normalize([0.485, 0.456, 0.406], [0.229, 0.224, 0.225]),
+            ])
+
+            dataset = datasets.ImageFolder(str(self._training_dir), transform=train_transform)
+            loader = DataLoader(dataset, batch_size=batch_size, shuffle=True, num_workers=2)
+
+            model = models.mobilenet_v3_small(weights=models.MobileNet_V3_Small_Weights.DEFAULT)
+            model.classifier[-1] = nn.Linear(model.classifier[-1].in_features, num_classes)
+
+            device = torch.device("cuda" if torch.cuda.is_available() else "cpu")
+            model = model.to(device)
+
+            for param in model.features.parameters():
+                param.requires_grad = False
+
+            optimizer = torch.optim.Adam(model.classifier.parameters(), lr=1e-3)
+            criterion = nn.CrossEntropyLoss()
+
+            for epoch in range(epochs):
+                model.train()
+                running_loss = 0.0
+                correct = 0
+                total = 0
+
+                if epoch == 5:
+                    for param in model.features.parameters():
+                        param.requires_grad = True
+                    optimizer = torch.optim.Adam(model.parameters(), lr=1e-4)
+
+                for inputs, labels in loader:
+                    inputs, labels = inputs.to(device), labels.to(device)
+                    optimizer.zero_grad()
+                    outputs = model(inputs)
+                    loss = criterion(outputs, labels)
+                    loss.backward()
+                    optimizer.step()
+
+                    running_loss += loss.item()
+                    _, predicted = outputs.max(1)
+                    total += labels.size(0)
+                    correct += predicted.eq(labels).sum().item()
+
+                accuracy = correct / total if total > 0 else 0
+                self.status.epoch = epoch + 1
+                self.status.progress = (epoch + 1) / epochs
+                self.status.accuracy = accuracy
+                log.info("Epoch %d/%d — loss: %.4f, accuracy: %.4f",
+                         epoch + 1, epochs, running_loss / len(loader), accuracy)
+
+            if self._model_output_path.exists():
+                backup_path = self._model_output_path.with_suffix(".backup.pt")
+                shutil.copy2(self._model_output_path, backup_path)
+                log.info("Backed up previous model to %s", backup_path)
+
+            model = model.to("cpu")
+            torch.save(model, self._model_output_path)
+            log.info("Pet ID model saved to %s (accuracy: %.2f%%)",
+                     self._model_output_path, self.status.accuracy * 100)
+
+            self.status.is_training = False
+            return True
+
+        except Exception as e:
+            log.exception("Training failed: %s", e)
+            self.status.error = str(e)
+            self.status.is_training = False
+            return False