This repository implements Stwo, a high-performance STARK proof system based on Circle STARKs (ePrint 2024/278) and the Stwo whitepaper. It is a Rust library providing a prover and verifier for zero-knowledge proofs over the Mersenne prime field M31 (p = 2^31 − 1) with extension fields CM31 and QM31. The architecture is modular: a core crate defines field arithmetic, FRI, polynomial commitment, and verification logic; a prover crate provides backend-agnostic proving with CPU and SIMD (AVX2/AVX-512, NEON, WASM) implementations; a constraint-framework crate offers a DSL for building AIR constraints and lookup arguments (LogUp, GKR). The system supports multiple hash channels (Blake2s, Poseidon252, Keccak256) and includes a no_std verifier suitable for on-chain deployment. Key entry points are `prover::prove()` and `core::verifier::verify()`. External dependencies include blake2, blake3, sha3, keccak, and starknet-crypto. The codebase is used in production by StarkWare's SHARP shared prover and by the Cairo VM prover (stwo-cairo). It demonstrates exceptional craftsmanship in SIMD optimization, field arithmetic, and protocol implementation, though it relies on nightly Rust and contains some unsafe code for performance.