GEO (Gene Expression Omnibus) is a gene expression data repository that accepts array- and sequence-based data. It is developed and maintained by NCBI (National Center for Biotechnology Information). It has become very difficult to grasp the outline of data accepted by GEO, since GEO accepts a variety of gene expression data, (the accepted data are categorized into three types:Datasets derived from research projects, Samples used and Platform to produce the data) that has been generated using different experimental methods.To ease this situation, Dr. Okubo and colleagues at the National Institute of Genetics (Japan) have developed the Gene Expression Omnibus (GEO) Overview. It is now maintained by the Database Center for Life Science (DBCLS). This tutorial describes how to use the "Gene Expression Omnibus (GEO) Overview" . It should be helpful when one wants to find out the outline of the abundant gene expression data available from GEO.