# Introduction

## What is Phat Contract#

Phat Contract is the innovative programming model enabling Off-chain Computation. It’s also known as Fat Contract as a practice of the “Fat Protocol & Thin Application” concept, and for its rich functionalities compared with existing smart contract. Phat Contract uses Rust-based ink! language.

Although with the name of “contract”, Phat Contract is more like the Web3 version of Amazon Lambda backed by the decentralized Phala computing cloud, and meant to support complex computation with low latency and cost.

Phat Contract is not meant to replace the smart contracts, instead, it tries to be the missing decentralized computation unit for them. For example, instead of implementing an ERC-20 token with Phat Contract (whose balance has to be stored on-chain), we recommend to deploy your ERC-20 contract on Ethereum, and use Phat Contract to operate it.

Join our Discord server and learn more in the #dev channel.

## When will you need it#

When a single smart contract is not ENOUGH for your DApp, implement the rest logic with Phat Contract.

Since the on-chain storage and execution can be expensive (from both monetary and performance perspectives), it’s reasonable to keep the on-chain contract small and compact, and implement the other logic elsewhere. The existing DApps usually implement their own backend logic as normal programs and deploy them to centralized services like AWS (Amazon Web Services).

With the computing service from Phala and its Phat Contract, you can run your backend programs on a decentralized infrastructure with privacy, performance and low cost.