The Gene Ontology (GO) project, a collaborative effort involving major model organism databases and bioinformatics resource centers, develops and uses structured, controlled vocabularies for annotating genes, gene products, and sequences. The GO Consortium continuously improves the vocabulary content, incorporating community input through various mechanisms. A growing number of model organism databases and genome annotation groups contribute annotation sets using GO terms to GO's public repository. Updates to the AmiGO browser have enhanced access to contributed genome annotations. As the GO project expands, its use becomes more varied and widespread, enabling biologists to infer knowledge from large datasets. The GO annotations are particularly useful for interpreting microarray results and facilitating the organization and comparison of biological information across different clades. The GO Consortium manages content changes centrally, involving experts in various biomedical fields to ensure consistency and accuracy. The project also supports new groups seeking to use GO for gene product annotation and makes the resulting data publicly available. GO annotations are now available for over 30 genomes and numerous additional species. The GO Consortium holds Annotation Camps to refine annotation practices and improve consistency. Software tools like DAG-Edit and OBO-Edit facilitate the use and manipulation of GO terms and annotations. GO data are used in various applications, including gene expression analysis, gene function prediction, and association with genetically inherited diseases. The GO project is also integrated into the NCI cancer biomedical informatics grid (caBIG) initiative, enabling the cancer community to analyze microarray and proteomic data. The GO project is supported by grants from the NIH/NHGRI and the European Union RTD Programme.The Gene Ontology (GO) project, a collaborative effort involving major model organism databases and bioinformatics resource centers, develops and uses structured, controlled vocabularies for annotating genes, gene products, and sequences. The GO Consortium continuously improves the vocabulary content, incorporating community input through various mechanisms. A growing number of model organism databases and genome annotation groups contribute annotation sets using GO terms to GO's public repository. Updates to the AmiGO browser have enhanced access to contributed genome annotations. As the GO project expands, its use becomes more varied and widespread, enabling biologists to infer knowledge from large datasets. The GO annotations are particularly useful for interpreting microarray results and facilitating the organization and comparison of biological information across different clades. The GO Consortium manages content changes centrally, involving experts in various biomedical fields to ensure consistency and accuracy. The project also supports new groups seeking to use GO for gene product annotation and makes the resulting data publicly available. GO annotations are now available for over 30 genomes and numerous additional species. The GO Consortium holds Annotation Camps to refine annotation practices and improve consistency. Software tools like DAG-Edit and OBO-Edit facilitate the use and manipulation of GO terms and annotations. GO data are used in various applications, including gene expression analysis, gene function prediction, and association with genetically inherited diseases. The GO project is also integrated into the NCI cancer biomedical informatics grid (caBIG) initiative, enabling the cancer community to analyze microarray and proteomic data. The GO project is supported by grants from the NIH/NHGRI and the European Union RTD Programme.