# Symbiosis Core Smart Contracts Overview

{% hint style="info" %}

* If you are curious to see the Symbiosis Protocol in action, check out \
  [Symbiosis WebApp](https://app.symbiosis.finance/swap?chainIn=Ethereum\&tokenIn=ETH).
* The Symbiosis Core Smart Contracts are open source and available at\
  <https://github.com/symbiosis-finance/core-contracts>.
* For security audits, please refer to [Security Audits](/main-concepts/security-audits.md).
  {% endhint %}

The Symbiosis Core Smart Contracts implement the on-chain logic for cross-chain operations. These smart contracts are deployed across all blockchain networks supported by Symbiosis and are fine-tuned to handle requests specific to each network. When the smart contracts process a request, they generate events called Oracle requests. These events contain all the necessary data to execute the next steps of cross-chain operations on other chains and are key triggers for the relayers’ activity.

The Symbiosis protocol has two slightly different sets of smart contracts: Scheme 1.

<figure><img src="/files/bztrEid4nS0scNvf29Bu" alt=""><figcaption><p>Scheme 1. The Symbiosis Core Contracts.</p></figcaption></figure>

Let's see what each contract does:

1. The **MetaRouterGateway** contract secures the interactions of the MetaRouter contract with users’ ERC20 tokens. <mark style="background-color:purple;">Users’ ERC20 tokens should only be approved for this contract.</mark>&#x20;
2. The **MetaRouter** contract manages calls to other Symbiosis contracts on one blockchain within a  cross-chain operation.&#x20;
3. The **Portal** contract locks and unlocks users' stablecoins during cross-chain operations.
4. The **BridgeV2** contract is a proxy between the Symbiosis Relayers Network and the Synthesis/Portal contracts.
5. The **Synthesis** contract mints and burns sTokens during cross-chain operations.&#x20;
6. A **Symbiosis Octopool** is an AMM liquidity pool containing several types of tokens of the same nominal value. For more information, see [Symbiosis Octopools](/crosschain-liquidity-engine/symbiosis-octopools.md).&#x20;
7. The **MulticallRouter** contract manages calls to Symbiosis contracts on on the Symbiosis Host Chain within a cross-chain operation, when the Symbiosis Host Chain isn't the source or destination blockchain of the cross-chain operation.

[Symbiosis Routing Contracts](/crosschain-liquidity-engine/symbiosis-routing-contracts.md) document explains how the Core Smart Contracts interact during cross-chain operations.&#x20;

## More Information

* sTokens (sStable tokens, sWETH, sWBTC, etc.) are explained here: [Symbiosis sTokens and Supported Chains](/main-concepts/wrapped-tokens.md).
* Here is an explanation of how the Symbiosis Protocol handles cross-chain operations [Symbiosis Routing Contracts](/crosschain-liquidity-engine/symbiosis-routing-contracts.md).


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