> For the complete documentation index, see [llms.txt](https://optim-finance.gitbook.io/optim-finance/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://optim-finance.gitbook.io/optim-finance/leviathan/english/core-concepts/sequencers/role-in-layer-2-rollups.md).

# Role in Layer 2 Rollups

In Optimistic Rollups, sequencers assume transactions are valid by default and post them to the Layer 1 blockchain. The transactions are considered final after a challenge period, during which they can be contested for fraud.

In ZK Rollups, sequencers aggregate and execute transactions off-chain and then generate a cryptographic proof (a zero-knowledge proof) that attests to the validity of these transactions. This proof is then posted to the main chain, where it is verified, ensuring the correctness of the off-chain execution without revealing the transaction data.