An Optimistic Rollup is a type of L2 scaling system that incorporates fault proofs to incentivize honest behavior from the Sequencer. Rollups provide scalability to blockchains by providing off-chain computation and, in some cases, off-chain state storage. Optimistic rollups differ from Plasma-based scaling systems in that they store some transaction data on the corresponding layer one blockchain so that the layer one nodes, rather than just the rollup nodes, can verify the transaction on the rollup. In contrast, Plasma systems typically keep all the transaction data and computation off-chain. Optimistic rollups differ from zero-knowledge and validity proof-based rollups in their 'optimistic' assumption, whereby the former relies on actors to identify and report erroneous rollup transactions through fault proofs, typically introducing a challenge period of up to two weeks during which observers can submit fault proofs. In contrast, the latter uses validity proofs to prove that transactions are valid without needing any external reporting.