Stockfish is an open-source chess engine that has ranked as the strongest or near-strongest engine in independent tests for many years. It is the backbone of game analysis on Lichess, Chess.com, and countless other platforms, as well as the default engine in most chess GUIs.

The engine uses a traditional NNUE (efficiently updatable neural network) evaluation function combined with highly optimized alpha-beta search. It can analyze millions of positions per second on modern hardware, evaluates positions with extreme depth, and handles all standard chess rules including Chess960. Regular updates from its open development community continually push its strength further.

Stockfish is free to download and use. It runs on Windows, macOS, and Linux, and integrates with any UCI-compatible chess GUI such as Arena, Scid vs PC, or ChessBase. The source code is available on GitHub under the GPL license. For players wanting to analyze their games, Stockfish is the reference-level tool.