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Optimistic Rollups
How they work: Transactions are processed off-chain and submitted to L1 as compressed calldata. The rollup "optimistically" assumes all transactions are valid. A 7-day challenge period allows anyone to submit a fraud proof if a transaction is invalid.
Benefits: Full EVM compatibility (same Solidity code, same tooling). Mature (Arbitrum launched 2021, Optimism 2021, Base 2023). Large TVL.
Drawbacks: 7-day withdrawal period (to allow fraud proofs). Fast bridges are available but add trust assumptions.
Leaders: Arbitrum One ($10B+ TVL), Optimism ($5B+ TVL), Base (Coinbase).
ZK Rollups
How they work: Each batch of transactions generates a cryptographic ZK proof (zk-SNARK or zk-STARK) that is mathematically verified on L1. If the proof is valid, the L1 accepts the batch as correct — no challenge period needed.
Benefits: Near-instant finality (proof verified on L1 immediately). No 7-day withdrawal period. Cryptographically stronger security guarantee.
Drawbacks: Full EVM compatibility has been technically challenging (ZK provers were historically not EVM-compatible). Now largely solved: zkSync Era, Polygon zkEVM, and Scroll all offer full EVM equivalence. Higher proof generation cost (being reduced rapidly with hardware acceleration and proof system improvements).
Leaders: zkSync Era, Starknet, Polygon zkEVM, Scroll.
Development Decision
Use optimistic rollup when: Deploying existing Solidity code without modification. Accessing the Arbitrum or Optimism DeFi ecosystem. Development team is optimistic-rollup native.
Use ZK rollup when: Fast finality is required (bridging, high-frequency applications). Application requires the mathematical certainty of ZK proofs. Building on zkSync or Starknet's native toolchain.