Optimistic Rollups

Optimistic rollups assume batches posted to Ethereum are valid unless someone proves otherwise. Arbitrum and Optimism both use this model. The name reflects the default: optimism that operators act honestly, with a challenge mechanism as the backstop.

The challenge window is why optimistic rollup withdrawals take days rather than minutes. Your exit must wait until the batch containing your withdrawal is considered final — no one can still challenge it.

Optimistic rollups are simpler to build than ZK rollups because they do not require generating cryptographic proofs for every batch. The tradeoff is latency on exits and reliance on at least one honest watcher to catch fraud.