A rollup processes transactions off the main chain and posts compressed data back. The name comes from rolling up many transactions into a single batch. That batch is submitted to Layer 1, where it inherits the base chain's security. Users get lower fees because the cost of one batch is shared across hundreds of transactions. Rollups are the dominant Layer 2 scaling solution for Ethereum.

Two main types exist. Optimistic rollups assume transactions are valid and allow a challenge period for fraud proofs. If someone submits a fraudulent batch, anyone can prove it and earn a reward. Arbitrum and Optimism use this model. ZK rollups generate cryptographic proofs that the batch is valid. No challenge period is needed because the math proves correctness. ZK rollups are more complex and expensive to build, but they offer faster finality. Both types have grown rapidly. Most Ethereum activity now happens on rollups, not on mainnet. The mainnet is becoming a settlement layer for rollup data.

Rollup types

• Optimistic rollup — fraud proofs, challenge period, cheaper to build
• ZK rollup — validity proofs, faster finality, more complex
• General-purpose — supports any smart contract
• Application-specific — optimized for one use case

Rollups are not a temporary fix. They are the scaling roadmap for Ethereum. The base chain secures the data. The rollup handles the execution.