The Natural Language Toolkit (NLTK) is a comprehensive, open source Python library designed for human language data processing. It offers user-friendly interfaces to over 50 corpora and lexical resources, such as WordNet, along with a suite of text processing libraries for tasks including classification, tokenization, stemming, tagging, parsing, and semantic reasoning. NLTK also provides wrappers for industrial-strength NLP libraries and maintains an active discussion forum. Accompanied by a hands-on guide that introduces programming fundamentals alongside computational linguistics topics, and comprehensive API documentation, NLTK is suitable for linguists, engineers, students, educators, researchers, and industry professionals. It is compatible with Windows, Mac OS X, and Linux platforms. Notably, NLTK is a free, community-driven project.