fairseq is an open-source sequence modeling toolkit from Facebook AI Research (FAIR), implemented in Python on top of PyTorch. It provides reference implementations for a wide range of sequence models (Transformer, LSTM, Conv, wav2vec, wav2vec 2.0, etc.) and supports tasks such as machine translation, summarization, language modeling, and speech processing. Key features include multi-GPU and distributed training, fast generation (beam search, sampling, diverse beam), mixed-precision training, parameter/optimizer sharding, and many pre-trained models and examples. The project is MIT-licensed and documented at readthedocs.