Step 1: Define LightningModule (organize your PyTorch code)

class LitModel(L.LightningModule): def init(self, hidden_size=128): super().init() self.model = nn.Sequential( nn.Linear(28 * 28, hidden_size), nn.ReLU(), nn.Linear(hidden_size, 10) )

def training_step(self, batch, batch_idx):
    x, y = batch
    y_hat = self.model(x)
    loss = nn.functional.cross_entropy(y_hat, y)
    self.log('train_loss', loss)  # Auto-logged to TensorBoard
    return loss

def configure_optimizers(self):
    return torch.optim.Adam(self.parameters(), lr=1e-3)

Step 2: Create data

train_loader = DataLoader(train_dataset, batch_size=32)

Step 3: Train with Trainer (handles everything else!)

trainer = L.Trainer(max_epochs=10, accelerator='gpu', devices=2) model = LitModel() trainer.fit(model, train_loader)


**That's it!** Trainer handles:
- GPU/TPU/CPU switching
- Distributed training (DDP, FSDP, DeepSpeed)
- Mixed precision (FP16, BF16)
- Gradient accumulation
- Checkpointing
- Logging
- Progress bars

## Common workflows

### Workflow 1: From PyTorch to Lightning

**Original PyTorch code**:
```python
model = MyModel()
optimizer = torch.optim.Adam(model.parameters())
model.to('cuda')

for epoch in range(max_epochs):
    for batch in train_loader:
        batch = batch.to('cuda')
        optimizer.zero_grad()
        loss = model(batch)
        loss.backward()
        optimizer.step()

Lightning version:

class LitModel(L.LightningModule):
    def __init__(self):
        super().__init__()
        self.model = MyModel()

    def training_step(self, batch, batch_idx):
        loss = self.model(batch)  # No .to('cuda') needed!
        return loss

    def configure_optimizers(self):
        return torch.optim.Adam(self.parameters())

# Train
trainer = L.Trainer(max_epochs=10, accelerator='gpu')
trainer.fit(LitModel(), train_loader)

Benefits: 40+ lines → 15 lines, no device management, automatic distributed

Workflow 2: Validation and testing

class LitModel(L.LightningModule):
    def __init__(self):
        super().__init__()
        self.model = MyModel()

    def training_step(self, batch, batch_idx):
        x, y = batch
        y_hat = self.model(x)
        loss = nn.functional.cross_entropy(y_hat, y)
        self.log('train_loss', loss)
        return loss

    def validation_step(self, batch, batch_idx):
        x, y = batch
        y_hat = self.model(x)
        val_loss = nn.functional.cross_entropy(y_hat, y)
        acc = (y_hat.argmax(dim=1) == y).float().mean()
        self.log('val_loss', val_loss)
        self.log('val_acc', acc)

    def test_step(self, batch, batch_idx):
        x, y = batch
        y_hat = self.model(x)
        test_loss = nn.functional.cross_entropy(y_hat, y)
        self.log('test_loss', test_loss)

    def configure_optimizers(self):
        return torch.optim.Adam(self.parameters(), lr=1e-3)

# Train with validation
trainer = L.Trainer(max_epochs=10)
trainer.fit(model, train_loader, val_loader)

# Test
trainer.test(model, test_loader)

Automatic features:

Validation runs every epoch by default
Metrics logged to TensorBoard
Best model checkpointing based on val_loss

Workflow 3: Distributed training (DDP)

# Same code as single GPU!
model = LitModel()

# 8 GPUs with DDP (automatic!)
trainer = L.Trainer(
    accelerator='gpu',
    devices=8,
    strategy='ddp'  # Or 'fsdp', 'deepspeed'
)

trainer.fit(model, train_loader)

Launch:

# Single command, Lightning handles the rest
python train.py

No changes needed:

Automatic data distribution
Gradient synchronization
Multi-node support (just set num_nodes=2)

Workflow 4: Callbacks for monitoring

from lightning.pytorch.callbacks import ModelCheckpoint, EarlyStopping, LearningRateMonitor

# Create callbacks
checkpoint = ModelCheckpoint(
    monitor='val_loss',
    mode='min',
    save_top_k=3,
    filename='model-{epoch:02d}-{val_loss:.2f}'
)

early_stop = EarlyStopping(
    monitor='val_loss',
    patience=5,
    mode='min'
)

lr_monitor = LearningRateMonitor(logging_interval='epoch')

# Add to Trainer
trainer = L.Trainer(
    max_epochs=100,
    callbacks=[checkpoint, early_stop, lr_monitor]
)

trainer.fit(model, train_loader, val_loader)

Result:

Auto-saves best 3 models
Stops early if no improvement for 5 epochs
Logs learning rate to TensorBoard

Workflow 5: Learning rate scheduling

class LitModel(L.LightningModule):
    # ... (training_step, etc.)

    def configure_optimizers(self):
        optimizer = torch.optim.Adam(self.parameters(), lr=1e-3)

        # Cosine annealing
        scheduler = torch.optim.lr_scheduler.CosineAnnealingLR(
            optimizer,
            T_max=100,
            eta_min=1e-5
        )

        return {
            'optimizer': optimizer,
            'lr_scheduler': {
                'scheduler': scheduler,
                'interval': 'epoch',  # Update per epoch
                'frequency': 1
            }
        }

# Learning rate auto-logged!
trainer = L.Trainer(max_epochs=100)
trainer.fit(model, train_loader)

When to use vs alternatives

Use PyTorch Lightning when:

Want clean, organized code
Need production-ready training loops
Switching between single GPU, multi-GPU, TPU
Want built-in callbacks and logging
Team collaboration (standardized structure)

Key advantages:

Organized: Separates research code from engineering
Automatic: DDP, FSDP, DeepSpeed with 1 line
Callbacks: Modular training extensions
Reproducible: Less boilerplate = fewer bugs
Tested: 1M+ downloads/month, battle-tested

Use alternatives instead:

Accelerate: Minimal changes to existing code, more flexibility
Ray Train: Multi-node orchestration, hyperparameter tuning
Raw PyTorch: Maximum control, learning purposes
Keras: TensorFlow ecosystem

Common issues

Issue: Loss not decreasing

Check data and model setup:

# Add to training_step
def training_step(self, batch, batch_idx):
    if batch_idx == 0:
        print(f"Batch shape: {batch[0].shape}")
        print(f"Labels: {batch[1]}")
    loss = ...
    return loss

Issue: Out of memory

Reduce batch size or use gradient accumulation:

trainer = L.Trainer(
    accumulate_grad_batches=4,  # Effective batch = batch_size × 4
    precision='bf16'  # Or 'fp16', reduces memory 50%
)

Issue: Validation not running

Ensure you pass val_loader:

# WRONG
trainer.fit(model, train_loader)

# CORRECT
trainer.fit(model, train_loader, val_loader)

Issue: DDP spawns multiple processes unexpectedly

Lightning auto-detects GPUs. Explicitly set devices:

# Test on CPU first
trainer = L.Trainer(accelerator='cpu', devices=1)

# Then GPU
trainer = L.Trainer(accelerator='gpu', devices=1)

Advanced topics

Callbacks: See references/callbacks.md for EarlyStopping, ModelCheckpoint, custom callbacks, and callback hooks.

Distributed strategies: See references/distributed.md for DDP, FSDP, DeepSpeed ZeRO integration, multi-node setup.

Hyperparameter tuning: See references/hyperparameter-tuning.md for integration with Optuna, Ray Tune, and WandB sweeps.

Hardware requirements

CPU: Works (good for debugging)
Single GPU: Works
Multi-GPU: DDP (default), FSDP, or DeepSpeed
Multi-node: DDP, FSDP, DeepSpeed
TPU: Supported (8 cores)
Apple MPS: Supported

Precision options:

FP32 (default)
FP16 (V100, older GPUs)
BF16 (A100/H100, recommended)
FP8 (H100)

Resources

Docs: https://lightning.ai/docs/pytorch/stable/
GitHub: https://github.com/Lightning-AI/pytorch-lightning ⭐ 29,000+
Version: 2.5.5+
Examples: https://github.com/Lightning-AI/pytorch-lightning/tree/master/examples
Discord: https://discord.gg/lightning-ai
Used by: Kaggle winners, research labs, production teams

pytorch-lightning

PyTorch Lightning - High-Level Training Framework

Quick start