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A rollup executes transactions on a Layer 2 and posts the transaction data (and a state commitment) back to Layer 1.

The L1 stores enough information to reconstruct and verify the L2 state, which is what gives rollups their security.

There are two dominant rollup families:

• Optimistic Rollups — assume transactions are valid, allow challenges
• ZK Rollups — prove validity with cryptographic proofs

They differ in how they convince L1 the L2 state is correct.

Optimistic rollups assume every batch is valid by default — hence optimistic.

They post the new state to L1 without proving it immediately, relying on the threat of fraud proofs to keep operators honest.

After a batch is posted, a challenge window (often ~7 days) opens. Anyone can submit a fraud proof showing the batch was invalid.

If a valid challenge succeeds, the bad batch is reverted and the dishonest operator is penalized.

The challenge window means withdrawing from an optimistic rollup back to L1 takes time (often a week).

Liquidity providers offer instant withdrawals for a fee by fronting the funds during the wait.

ZK rollups generate a cryptographic validity proof (a zero-knowledge proof) for each batch.

L1 verifies the proof mathematically, so there is no need to trust the operator or wait for a challenge period.

A validity proof (such as a SNARK or STARK) demonstrates that the new state was computed correctly from the old state and the transactions.

If the proof verifies, the state is definitively correct — no challenges needed.

Because ZK proofs are verified immediately, ZK rollups offer fast finality and quick withdrawals.

The trade-off is that generating proofs is computationally expensive and historically harder for general smart contracts.

Comparing the two:

• Optimistic: EVM-compatible, cheaper proving, slow withdrawals
• ZK: instant finality, fast withdrawals, complex proving

Both post data to L1, so both inherit Ethereum security.

Rollups must publish transaction data so anyone can reconstruct the state — this is data availability.

Posting data to L1 (calldata or blobs) is a major cost; innovations like EIP-4844 blobs made it much cheaper.

Rollups execute off-chain and settle on L1. Optimistic rollups assume validity and use fraud proofs; ZK rollups prove validity cryptographically. Data availability anchors their security.

Next we look at moving assets between layers with bridges.

Test your rollup knowledge.

You learned that:

• Rollups execute off-chain and settle data on L1
• Optimistic rollups use fraud proofs + a challenge window
• ZK rollups use validity proofs for instant finality
• Data availability on L1 anchors security

Next: bridging assets between layers.