The Gene Expression Omnibus (GEO) database is an international public repository that archives and distributes high-throughput gene expression and functional genomics data sets. Created in 2000, GEO has evolved to include data from various technologies, such as DNA microarrays, protein or tissue arrays, high-throughput nucleic acid sequencing, SAGE, and RT-PCR. The database supports community-derived reporting standards and provides access to tens of thousands of studies, along with web-based tools for data visualization and analysis. This chapter details methods for querying and downloading GEO data, as well as using the analysis and visualization tools. GEO's homepage is at <http://www.ncbi.nlm.nih.gov/geo/>. The database is supported by the National Center for Biotechnology Information (NCBI) at the National Library of Medicine (NLM) and is free to use for data submission, download, and analysis.The Gene Expression Omnibus (GEO) database is an international public repository that archives and distributes high-throughput gene expression and functional genomics data sets. Created in 2000, GEO has evolved to include data from various technologies, such as DNA microarrays, protein or tissue arrays, high-throughput nucleic acid sequencing, SAGE, and RT-PCR. The database supports community-derived reporting standards and provides access to tens of thousands of studies, along with web-based tools for data visualization and analysis. This chapter details methods for querying and downloading GEO data, as well as using the analysis and visualization tools. GEO's homepage is at <http://www.ncbi.nlm.nih.gov/geo/>. The database is supported by the National Center for Biotechnology Information (NCBI) at the National Library of Medicine (NLM) and is free to use for data submission, download, and analysis.