Optimistic Rollups are a scaling technology that makes Ethereum faster and cheaper to use. They move most transaction activity off Ethereum mainnet, while still relying on Ethereum to keep everything secure.
The Basic Idea
Instead of processing every transaction directly on Ethereum, Optimistic Rollups:
Process transactions off chain
Bundle many transactions together
Send the results back to Ethereum
Ethereum assumes these transactions are valid unless proven otherwise. This assumption is why they’re called optimistic.
Why This Matters
Optimistic Rollups help solve two of Ethereum’s biggest challenges:
High transaction fees
Slow confirmation times
By handling activity off chain, these networks reduce congestion and lower costs, while still using Ethereum as the final authority.
How Security Comes from Ethereum
Even though transactions happen off chain, Ethereum still enforces security through a system called fraud proofs.
Transaction data is published to Ethereum
Anyone can check the results
If something is incorrect, a fraud proof can be submitted
If a fraud proof succeeds, the incorrect transaction is reversed and the party that submitted it is penalized. This is how Optimistic Rollups stay secure while remaining low cost.
The Challenge Period
After transactions are posted to Ethereum, there is a short review window called a challenge period.
During this time:
Transactions can be disputed using fraud proofs
Withdrawals back to Ethereum mainnet may take longer
This delay mainly affects withdrawals to Ethereum. Most everyday activity on the rollup feels fast and immediate.
What Is a Sequencer?
Optimistic Rollups use a component called a sequencer.
The sequencer:
Orders transactions
Batches them together
Submits them to Ethereum
This is what allows rollups to be fast and inexpensive. Because the sequencer plays an operational role, rollups may have tradeoffs such as temporary delays if the sequencer is unavailable. Ethereum still acts as the backstop for security.
Networks That Use Optimistic Rollups
Popular Ethereum Layer 2 networks built with Optimistic Rollups include:
Base
Optimism
Arbitrum
These networks are compatible with Ethereum apps and wallets, but offer faster confirmations and lower fees.
Fees and Gas
You still pay gas fees when using Optimistic Rollups, usually in ETH. However, these fees are typically much lower than using Ethereum mainnet directly