What Is Arbitrum? The Optimistic Rollup That Became an Entire L2 Ecosystem

What Arbitrum actually is

Arbitrum is an Ethereum Layer 2 network built by Offchain Labs, a company founded in 2018 by Princeton researchers Ed Felten, Steven Goldfeder, and Harry Kalodner, who had been publishing academic work on rollup-style scaling before it became a mainstream term. Arbitrum One, the main network most people mean when they say “Arbitrum,” went live on mainnet in mid-2021 and opened to the public in August 2021. If you haven’t already, read What Are Layer 2s first; it covers what a rollup is and why optimistic rollups like Arbitrum exist at all. This guide assumes that background and goes into what’s specifically true of Arbitrum rather than L2s in general.

The short version of what makes Arbitrum distinctive: it’s usually the largest or second-largest Ethereum L2 by the money actually deposited and used on it, it’s an optimistic rollup rather than a ZK-rollup, and it isn’t just one network anymore. Through a framework called Orbit, Arbitrum’s underlying technology has been packaged so other teams can launch their own chains built on it, some tied closely to Arbitrum’s ecosystem and some fully independent.

The technical mechanism, in plain language

Arbitrum runs on a codebase called Nitro, a major rewrite that shipped in August 2022 and forms the technical basis for both Arbitrum One and every Orbit chain built since. Like any optimistic rollup, Arbitrum batches transactions off Ethereum’s main chain, assumes they’re valid, posts a compressed version back to Ethereum, and gives anyone a window (Arbitrum’s is typically around seven days) to submit a fraud proof if something in a batch was wrong. A later upgrade called Stylus added something genuinely distinctive: the ability for developers to write smart contracts in languages like Rust and C, compiled to WebAssembly, alongside the Solidity code every other Ethereum chain uses, which can make certain compute-heavy contracts meaningfully cheaper to run.

The ARB token: what it actually governs

ARB launched in March 2023 through an airdrop to past Arbitrum users, and its main job is governance of the Arbitrum DAO, which controls Arbitrum One and Arbitrum Nova (a second, cheaper chain aimed at high-volume use cases like gaming and social apps) along with a large treasury. ARB holders vote on protocol upgrades, treasury spending, grant programs, and the DAO’s Security Council, a group with emergency powers to intervene if something goes seriously wrong.

It’s worth being precise about what ARB does not do. Owning ARB doesn’t give you a claim on Arbitrum One’s transaction fees the way owning stock gives you a claim on a company’s profits; there’s no automatic revenue-sharing or token burn tied to network usage. And ARB doesn’t automatically govern every chain built with Arbitrum’s technology. Orbit chains can launch with their own separate token and their own governance, fully independent of the Arbitrum DAO; some choose instead to license Arbitrum’s technology and compliance tooling through a program (Arbitrum’s DAO calls this its expansion program) that routes a share of their revenue back to the DAO treasury ARB holders control. So ARB is closer to a vote on the core Arbitrum One and Nova chains, plus a claim on a piece of a growing licensing business, than it is a token that governs one unified network.

From one chain to an ecosystem: Orbit

Orbit is the framework that lets a team spin up its own Layer 2 or Layer 3 chain using Arbitrum’s Nitro technology, choosing its own gas token, its own degree of decentralization, and increasingly, its own compliance settings; a 2026 upgrade added optional transaction-screening tools that individual Orbit chains can turn on if they need to meet specific regulatory requirements, which Arbitrum One itself does not use by default. This is the same basic idea behind Optimism’s OP Stack, covered in What Is Optimism: a rollup’s own technology gets repackaged as a toolkit other teams build on top of, rather than staying a single, closed network. Arbitrum’s Orbit ecosystem has attracted a mix of gaming chains, DeFi-specific chains, and, more recently, chains built by traditional financial firms exploring tokenized assets, though as with any young ecosystem, most individual Orbit chains carry far less activity than Arbitrum One itself.

Where Arbitrum actually stands today

Arbitrum has spent most of its history as the largest Ethereum L2 by value locked, but that lead has gotten genuinely competitive. Depending on which metric you use, either Arbitrum or Coinbase’s Base (built on Optimism’s OP Stack, not Arbitrum’s) currently holds the top spot, with the two of them together accounting for a large majority of all value held across dozens of active rollups; Base tends to lead on raw daily transaction and user counts, while Arbitrum has more often led on total value deposited. That’s a real shift from Arbitrum’s earlier, more clear-cut lead, and it’s worth checking current data yourself rather than assuming either network’s position is fixed, since rankings between the two have moved multiple times within a single year.

Strengths and limitations, honestly

Arbitrum’s genuine strengths are a long operating track record relative to most L2s, broad compatibility with existing Ethereum tools and contracts, and a technology base (Nitro and Orbit) that’s proven flexible enough to power both a general-purpose chain and a growing set of specialized ones. Its genuine limitations mirror those of optimistic rollups generally: native withdrawals back to Ethereum take roughly a week because of the fraud-proof challenge window, and the ARB token itself doesn’t capture network fee revenue directly, which is a real criticism of its long-term value proposition independent of how much activity happens on the network. Competition from Base and from ZK-rollups, which settle faster, has also made “biggest L2” a genuinely contested claim rather than a settled fact.

How to actually trade or hold it

ARB is available for spot trading on most major exchanges; check our exchange comparisons for current listings and fees where you live. If you’re new to buying crypto directly, Spot Trading Explained covers order types and custody basics first. Holding ARB and using the Arbitrum network are two separate things: you can use Arbitrum One’s apps and pay its (typically very low) gas fees without ever owning ARB itself, since gas on Arbitrum is paid in ETH, not ARB.

For the current price, market cap, and 24-hour chart, see our live Arbitrum page.

Frequently asked questions

Is Arbitrum a single blockchain or several? Both, in a sense. Arbitrum One and Arbitrum Nova are the two main chains run under the Arbitrum DAO’s governance. Orbit chains are separate networks built using the same underlying technology, some closely tied to the Arbitrum ecosystem and others fully independent with their own token and rules.

Do I need ARB to use Arbitrum? No. Gas fees on Arbitrum One are paid in ETH, not ARB. ARB is a governance and treasury token; you only need it if you want a say in how the Arbitrum DAO is run, not to send transactions or use apps on the network.

How is Arbitrum different from Optimism? Both are optimistic rollups with similar core mechanics, built by different teams (Offchain Labs and OP Labs, respectively) with different token designs and different approaches to letting others build on their technology. What Is Optimism covers Optimism’s side of that comparison, including its OP Stack and Superchain plans.

Is Arbitrum still the biggest Ethereum Layer 2? It’s one of the two largest, alongside Base, and which one leads depends on the specific metric and the moment you check. Both have separated meaningfully from the rest of the field, but “Arbitrum is the biggest L2” is no longer the uncontested claim it was in Arbitrum’s earlier years.

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