Optimistic rollups assume all transactions are valid by default (hence 'optimistic'), posting state updates to Layer 1 without immediate proof of correctness. Security comes from a challenge period, typically 7 days, during which anyone can submit a fraud proof if they detect an invalid state transition. If a fraud proof succeeds, the invalid transaction is reverted and the malicious sequencer is penalized. If no challenge occurs during the window, the state becomes final. This design is simpler and cheaper than zero-knowledge proofs, making optimistic rollups easier to build and more compatible with existing Ethereum tools. The EVM equivalence of Arbitrum and Optimism means Ethereum applications deploy with minimal modification. The primary trade-off is the withdrawal delay: moving assets from L2 to L1 requires waiting out the challenge period (or using third-party bridges that accept the fraud risk for faster exits, charging fees for the service). Arbitrum One and Optimism are the largest optimistic rollups, collectively hosting billions in TVL. Recent developments include multiple proving systems and shorter effective withdrawal times through improved bridge designs. Optimistic rollups benefit from relative simplicity but may eventually cede ground to ZK rollups as zero-knowledge proof technology matures.
Back to Glossary