The first part of Eth2 to ship will be the beacon chain. It won’t be fully operational right away because there won’t be any shard chains, so there’ll be nothing to keep in sync other than itself. Remember, the beacon chain’s main role in Eth2 is making sure all the shards have the most up-to-date data.

At first, the beacon chain will be responsible for registering validators and coordinating everyone’s staked ETH. This is foundational to the rest of Eth2 and lays the groundwork for the shard chains.

The beacon chain receives block attestations from shards and uses Casper to ensure they are finalised.

Prior to that, the shard blocks go through a proof-of-stake process:

• proposed by a chosen validator in a committee

• attested by the rest of the committee during an epoch “slot”

• crosslinked with the beacon chain after enough attestations

• finalized by Casper using cryptoeconomic incentives

And that’s how Eth2 stays in sync using the beacon chain.

Phase 1 is when we’ll meet the shard chains. These are chains that are delegated a portion of Ethereum’s transactions and account data. They will be proof-of-stake chains, which means validators (who have staked ETH) will create blocks – not miners like today.

We expect to start Phase 1 with 64 shards but they won’t support accounts or smart contracts right away.

Up until Phase 1.5, the Ethereum we use today on mainnet will continue as a proof-of-work blockchain. Transactions will continue to be processed by miners. But in Phase 1.5, mainnet will officially become a shard and transition to proof-of-stake.

For end-users and apps, this change should be seamless

In Phase 2, shards should be fully functional chains. Shards will now be compatible with smart contracts and they’ll be able to communicate with each other more freely. Developers may even be able to design shards in their own ways.