Biological databases play a central role in bioinformatics. They offer scientists the opportunity to access sequence and structure data for tens of thousands of sequences from a broad range of organisms. This unit provides a brief overview of major sequence databases and portals, such as GenBank, UCSC Genome Browser, and Ensembl. Model organism databases, including the TIGR Gene Indices, WormBase, and the Arabidopsis Information Resource (TAIR) are also covered. Non-sequence-centric databases such as Online Mendelian Inheritance in Man (OMIM), the Protein Data Bank (PDB), and the Kyoto Encyclopedia of Genes and Genomes (KEGG) are also discussed.