> ## Documentation Index
> Fetch the complete documentation index at: https://docs.optimism.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Interoperability

> One place to learn OP Stack interoperability, from the gentle introduction through message passing, safety, hands-on tutorials, and the normative spec.

<Info>OP Stack interop is in active development. Some features may be experimental.</Info>

OP Stack interoperability is a set of protocols that lets OP Stack chains
read each other's state, so a network of chains can feel like a single
blockchain: ETH and ERC-20 tokens move between chains by native minting and
burning, and contracts can compose with events from other chains at
low latency.

Interop is in active development and is being rolled out iteratively, so
read these materials as a picture of where the stack is going and what you
can already build against, not as a finished production system. Start with
the short introduction, then work through the mechanism pages in order:
they build from the message-passing model up to reorg safety and the
runtime that enforces it. The tutorials let you try everything on local or
devnet chains, and the spec is the normative definition throughout.

## Start here

* [OP Stack Interoperability](/op-stack/interop/interop): read this first
  for what interop enables (native asset transfers, cross-chain
  composability, horizontal scalability) and where to start building.

## Go deeper

* [OP Stack interoperability explainer](/op-stack/interop/explainer): the
  architecture end to end, covering initiating and executing messages,
  access-list validation, block safety levels, and dependency sets and
  clusters.
* [Interop message passing overview](/app-developers/guides/interoperability/message-passing):
  the messaging lifecycle at contract level, from the low-level
  `CrossL2Inbox` to the `L2ToL2CrossDomainMessenger` most apps use.
* [Reading logs with OP Stack interop](/app-developers/guides/interoperability/reading-logs):
  the pull model, where a contract validates a log from another chain
  directly instead of receiving a message.
* [Message expiration](/app-developers/guides/interoperability/message-expiration):
  why an unrelayed message eventually expires and how to reemit one that
  did.
* [Superchain ETH Bridge](/op-stack/interop/superchain-eth-bridge): how
  native ETH moves between chains through `SuperchainETHBridge` and the
  `ETHLiquidity` contract.
* [Interop reorg awareness](/op-stack/interop/reorg): how interop delivers
  low-latency messages without opening the door to cross-chain
  double-spends.
* [OP Supernode](/op-stack/interop/supernode): the runtime that hosts
  every chain in a dependency set in one process and performs the
  cross-chain verification work.

## Get hands-on

* [Build interoperable apps on OP Stack devnet](/app-developers/guides/interoperability/get-started):
  pick a development environment (local Supersim or the interop devnet)
  and start deploying.
* [Interop message passing tutorial](/app-developers/tutorials/interoperability/message-passing):
  build a cross-chain message passing system with the
  `L2ToL2CrossDomainMessenger` contract.
* [Relay transactions manually](/app-developers/tutorials/interoperability/manual-relay):
  construct and send an executing message yourself when no autorelayer
  does it for you.

## The normative spec

The definitive definition of interop behavior lives in the OP Stack
specifications:

* [Interop](https://specs.optimism.io/interop/overview.html?utm_source=op-docs\&utm_medium=docs):
  the root of the interop specs and the map of everything below it.
* [Messaging](https://specs.optimism.io/interop/messaging.html?utm_source=op-docs\&utm_medium=docs):
  the normative definitions of initiating and executing messages and
  their safety rules.
* [Predeploys](https://specs.optimism.io/interop/predeploys.html?utm_source=op-docs\&utm_medium=docs):
  the `CrossL2Inbox`, `L2ToL2CrossDomainMessenger`, and the other interop
  predeploy contracts.
* [SuperchainETHBridge](https://specs.optimism.io/interop/superchain-eth-bridge.html?utm_source=op-docs\&utm_medium=docs):
  the specification of native ETH bridging between interop chains.

## Audits and security

* [Crosschain security measures](/op-stack/security/interop-security):
  the trust model, what happens under sequencer equivocation, and the
  latency versus security tradeoff each chain configures.
* [Audit reports](/op-stack/security/audit-reports): the standing index of
  OP Stack security reviews, which routes to the full list in the
  monorepo.