Introducing Arbitrum One: our mainnet beta

Offchain Labs
4 min readMay 29, 2021

Arbitrum mainnet beta has launched for developers. What comes next? Read on.

Arbitrum has — — actually, wait. We’ll get to us in a second, but first let’s talk about you, our amazing community. You’re incredible. We’ve been blown away by your support and both humbled and honored that so many have entrusted us with our critical mission to scale Ethereum. Ethereum is bigger than any one team or project, and we’re truly fortunate to be a part of this extraordinary ecosystem. 💙

Alright, back to us for a sec.

Today’s the day! We’ve deployed the Arbitrum contracts on Ethereum mainnet, and have begun onboarding infrastructure and projects. The developer interest and enthusiasm for Arbitrum has exceeded our wildest expectations. Over 250 teams have requested access for our developer launch, and we can’t wait to see what they build on Arbitrum and how much gas savings this will enable.

So what does this all mean? How many teams have access and when can end users get in on the fun? Let’s dig into it.

Fair Launch

As we’ve announced previously, we’re committed to a fair launch in which projects of all shapes and sizes will be put on the same footing, and nobody will be given priority access. To facilitate this, Arbitrum One is initially open to all developers that have requested access, and we’ll open it up to end users once there’s a quorum of live projects that have deployed on the network and are ready to go live.

Operationally, with over 250 teams requesting to come online for the developer mainnet, it’s going to take us into next week before we can onboard everyone, but this process is underway. To make sure that nobody is disadvantaged by this, we will guarantee that all projects that have already applied and are granted access will have at least two weeks to build and test before we open Arbitrum One to everyone.

Also, we had several independent users request access, and while we genuinely love your enthusiasm and appreciate your support, we’re only open for development and testing now and we ask that you wait until we open for everyone.

Wait, isn’t it called Arbitrum? What’s Arbitrum One?

We’ve named our mainnet chain, Arbitrum One and have assigned a chainid of 0xa4b1 (decimal: 42161).

Arbitrum is the technology and Arbitrum One is our flagship chain. Our team is fully focussed on Arbitrum One, and all of our announced partners like Etherscan and Alchemy are supporting the Arbitrum One chain. But it’s possible that at some point in the future there will be other rollup chains that also use the Arbitrum technology, and to make sure this never gets confusing, we’ve differentiated the name of the tech (Arbitrum) from the name of the chain (Arbitrum One).

Confused? Let’s try again: Arbitrum One is our mainnet Ethereum L2 chain built using the Arbitrum technology.

Mainnet beta and the path to full decentralization

As we’ve discussed, for the first few months of mainnet, we’ll be maintaining the ability to upgrade and even pause the system if it becomes necessary, and we will be referring to this phase as mainnet beta. We will likely do some performance and feature upgrades during this period and will communicate more about this on our Discord.

While we’ve done significant security diligence, the ability to do fast upgrades for the first few months after launch is important in order to be able to respond quickly to any security events, and make sure that user funds remain safe at all times while everyone gets more experience with the system. We’ll also be operating the chain under a conservative speed limit during this phase.

We will be talking a lot more over the coming weeks about how we get from here to full decentralization, and we’re currently targeting the end of the summer to phase out these controls.


While Arbitrum reduces fees by more than 50x for most workloads that we’ve seen, Arbitrum (like all rollups) is not free. The majority of the fees collected on Arbitrum are used to pay for posting calldata on Ethereum (so they end up going to the Ethereum miners), and there are also costs to operate the chain and the supporting infrastructure. ETH is the native asset on Arbitrum, and fees will be paid in ETH on L2. For more info on fees, see here. In order to use the chain, users will first have to use the Arbitrum bridge to transfer ETH from Ethereum to their L2 wallet

What’s next?

Although Arbitrum One is only open to developers right now, users will be able to follow along, monitor the activity on the chain, and cheer on your favorite teams.

We’ve begun doing a series of announcements with Arbitrum ecosystem partners, and stay tuned for many more in the coming days. It’s an exciting period ahead, and we’re working hard to make sure that all of your favorite apps and infrastructure will be live and ready when we open Arbitrum One to all users.

Join our team

So you’ve read through the entire post — you could be the type of person we’re looking to hire! We have several openings — from crypto native developers to strong system developers that are looking to make their crypto debut. Apply to join us — you won’t regret it.