AmiGO is a web application that allows users to query, browse, and visualize ontologies and related gene product annotation data. It is available online at the Gene Ontology (GO) website and can also be downloaded and installed for local use. AmiGO is free open source software developed and maintained by the GO Consortium. It provides access to ontology information, gene product details, annotations, and various tools such as BLAST search, Term Enrichment, GO Slimmer, and the GO Online SQL Environment. AmiGO offers term information, including term details, lineage, and annotations. Gene product information includes details such as symbol, full name, synonyms, and database ID. Users can search for GO terms or gene products, and browse the GO ontology tree. The application supports various formats for viewing and saving tree information, including GraphViz, RDF-XML, and OBO. Other functionalities include BLAST search, Term Enrichment, GO Slimmer, and GOOSE (GO Online SQL Environment). BLAST allows users to search gene products in the GO database. Term Enrichment identifies significantly overrepresented terms in the context of the GO tree. GO Slimmer remaps granular annotations to high-level terms. GOOSE enables SQL queries on the GO database. AmiGO is a server-based Perl application that retrieves information from a database and processes it to produce web pages. It is open source and freely available for distribution and modification. AmiGO is under active development to keep pace with the growth of data availability. Future developments include integration of GO Reference Genome data, new ontology visualizations, and community-driven annotation through wiki-style software. User feedback is welcomed through feature request and bug trackers. The GO Consortium provides funding for AmiGO development. No conflicts of interest are declared.AmiGO is a web application that allows users to query, browse, and visualize ontologies and related gene product annotation data. It is available online at the Gene Ontology (GO) website and can also be downloaded and installed for local use. AmiGO is free open source software developed and maintained by the GO Consortium. It provides access to ontology information, gene product details, annotations, and various tools such as BLAST search, Term Enrichment, GO Slimmer, and the GO Online SQL Environment. AmiGO offers term information, including term details, lineage, and annotations. Gene product information includes details such as symbol, full name, synonyms, and database ID. Users can search for GO terms or gene products, and browse the GO ontology tree. The application supports various formats for viewing and saving tree information, including GraphViz, RDF-XML, and OBO. Other functionalities include BLAST search, Term Enrichment, GO Slimmer, and GOOSE (GO Online SQL Environment). BLAST allows users to search gene products in the GO database. Term Enrichment identifies significantly overrepresented terms in the context of the GO tree. GO Slimmer remaps granular annotations to high-level terms. GOOSE enables SQL queries on the GO database. AmiGO is a server-based Perl application that retrieves information from a database and processes it to produce web pages. It is open source and freely available for distribution and modification. AmiGO is under active development to keep pace with the growth of data availability. Future developments include integration of GO Reference Genome data, new ontology visualizations, and community-driven annotation through wiki-style software. User feedback is welcomed through feature request and bug trackers. The GO Consortium provides funding for AmiGO development. No conflicts of interest are declared.