What is rollup?

A blockchain that executes transactions away from a main chain in batches, then posts the data and a proof back to it, inheriting that chain's security.

Optimistic rollups assume a batch is valid and allow a challenge period, while zero-knowledge rollups post a validity proof. Most Ethereum layer 2 networks are rollups.

The word "inheriting" is doing the work in that definition, and it is where most misunderstanding lives. A rollup is not simply a faster chain running beside Ethereum. It posts its transaction data to Ethereum, so anyone can reconstruct its state independently, which is what distinguishes it from a sidechain with its own validator set and its own security.

How it works

1. A sequencer orders transactions. Users submit to the rollup, and a sequencer decides the order and returns a near-instant receipt. Most rollups run a single sequencer today, which is the main centralization point.
2. Execution happens off the main chain, so throughput is not limited by the main chain's block space.
3. Data is posted back. Compressed transaction data goes to Ethereum, historically as calldata and now usually in blobs introduced by the proto-danksharding upgrade, which cut the dominant cost for rollups (source: EIP-4844). See blob transaction and data availability.
4. Validity is established one of two ways. An optimistic rollup assumes correctness and allows anyone to submit a fraud proof during a challenge window, commonly seven days. A zk rollup posts a validity proof that the main chain verifies, so correctness is established immediately.

That difference is what you feel as a user. Withdrawing from an optimistic rollup to Ethereum through the canonical bridge means waiting out the challenge period. Withdrawing from a validity-proof rollup does not.

Example

Illustrative. Suppose a batch contains 1,000 transfers. Executed individually on Ethereum they would consume the main chain's block space 1,000 times. Rolled up, the execution happens off chain and only compressed data plus a proof reaches Ethereum, so the fixed cost is shared across all 1,000 users.

At an illustrative $2.00 of main chain cost per batch spread over 1,000 transfers, the data portion of each user's fee is a fifth of a cent. Add the rollup's own execution fee and the total is typically a small fraction of an Ethereum transaction. The saving comes from sharing a fixed cost, which is why rollup fees fall when usage rises rather than the reverse.

Why it matters when you buy

Rollups are where a large share of Ethereum activity now happens, so withdrawing an asset to the wrong network is a real risk: an exchange that supports a token on Ethereum may not support it on a particular rollup, and sending it there can lose it. Check the network selector before withdrawing. Sending crypto across chains covers the mechanics, layer 1 versus layer 2 covers the categories, and the chain pages compare measured costs.

Related terms

layer 2 — the broader category; optimistic rollup — challenge-period design; zk rollup — validity-proof design; sequencer — orders transactions, usually centralized; data availability — why posting data matters; canonical bridge — the official exit route.

Questions

Is a rollup as safe as Ethereum itself?

Not identically. It relies on Ethereum for data availability and settlement, which is a strong foundation, but adds its own risks: sequencer centralization, upgradeable contracts, and in some cases an admin key that can alter the system.

Why does withdrawing take a week?

On an optimistic rollup the challenge window is the security mechanism. It gives anyone time to prove a batch invalid. Validity-proof rollups establish correctness immediately and do not need the wait.

Can I send tokens from an exchange straight to a rollup?

Only if the exchange supports that network for that asset, and the network selector on the withdrawal form says so. Sending to an unsupported network is one of the most common ways people lose funds permanently.