What is a rollup, and how do optimistic and ZK rollups differ?
A rollup runs transactions away from Ethereum, then posts their data back to Ethereum in batches, so Ethereum keeps the record.1,2 Optimistic rollups assume each batch is right unless someone proves fraud, while ZK rollups prove every batch with cryptography.2
One batch, many users
Rollups bundle, or "roll up", hundreds of transactions into a single transaction on layer 1.2 The layer 1 fee is shared by everyone in the batch, so each user pays less.2 Rollups also squeeze the data down, so less of it has to be posted on Ethereum.3
The rollup runs the transactions itself, but posts the transaction data to Ethereum.2 Once that data is on Ethereum, reversing a rollup transaction would mean reversing Ethereum itself.2
Optimistic rollups: innocent until proven guilty
Optimistic rollups are called "optimistic" because they assume transactions are valid. They don't post a proof for each batch.3
After a batch is posted, there is a time window called the challenge period. During it, anyone can challenge the result with a fraud proof: evidence, checked on Ethereum, that the batch was worked out wrongly.3 If the fraud proof succeeds, the rollup re-runs the transactions and fixes its records, and the operator who included the bad transaction is penalised.3 The chain stays safe as long as there is at least one honest node checking it.3
The seven-day wait
Leaving an optimistic rollup through its own bridge means waiting for the challenge period to end. It lasts roughly seven days.3 To skip the wait, a user can go to a liquidity provider, who pays them on Ethereum straight away in exchange for a fee.3
Zero-knowledge (ZK) rollups: proof first
ZK rollups also run transactions in batches away from Ethereum. They post a summary of the changes, plus a validity proof.4 A validity proof is a piece of cryptography showing that the changes really are the result of running those transactions.4 A smart contract on Ethereum checks the proof before it accepts the rollup's new balances.4
Because the proof is checked first, there is no challenge delay when moving funds from a ZK rollup to Ethereum.4 The trade-off is cost: making and checking proofs is expensive, and making them needs special hardware.4
Side by side
| Optimistic rollup | ZK rollup | |
|---|---|---|
| How a batch is checked | Assumed valid; anyone can challenge it with a fraud proof3 | Comes with a validity proof that Ethereum checks4 |
| Leaving for Ethereum | Wait for the challenge period, roughly seven days3 | No delay once the proof is verified4 |
| What keeps it honest | At least one honest node3 | Cryptography, not honest people4 |
| Main drawback | Must post all transaction data, which can raise costs3 | Proofs are costly and need special hardware4 |
Who runs the rollup
Some rollups are run by a single operator, called a sequencer, that orders transactions and posts the batches.3,4 A sequencer can influence the order of transactions, and could refuse to include yours.3,4 As a safety net, rollups let users send transactions straight to the rollup's contract on Ethereum.3,4
Where the data goes
Since the Dencun upgrade, rollups can also post their data in blobs, which are usually cheaper than the older option, called calldata.4 The network only has to keep serving blob data for roughly 18 days, so blobs are not permanent storage.4
- Scaling. ethereum.org developer docs, updated 13 April 2026.
- What is layer 2?. ethereum.org, updated 23 July 2026.
- Optimistic rollups. ethereum.org developer docs, updated 23 June 2026.
- Zero-knowledge rollups. ethereum.org developer docs, checked 22 October 2026.
General education built from the publicly available sources listed above. Not financial, legal, tax or investment advice. Naming a company, project or regulator is not an endorsement either way. Last checked 2026-10-22. Terms and disclaimer.