The Antibiotic Resistance Genes Database (ARDB) is a manually curated database that unifies most publicly available information on antibiotic resistance. It provides detailed annotations for resistance genes, including resistance profiles, mechanisms of action, ontology, COG, and CDD annotations, as well as external links to sequence and protein databases. ARDB supports sequence similarity searches and includes a tool for characterizing common mutations that confer antibiotic resistance. The database contains information for 13,293 genes, 377 types, 257 antibiotics, 632 genomes, 933 species, and 124 genera. ARDB is available at http://ardb.cbcb.umd.edu/. The database was created to address the limitations of existing resources and to facilitate the identification and characterization of antibiotic resistance genes. It provides a centralized compendium of information on antibiotic resistance, facilitates the consistent annotation of resistance information in newly sequenced organisms, and aids in the identification and characterization of new genes. ARDB includes information on 632 complete bacterial genomes, allowing quick search and comparison of resistance profiles. ARDB includes ontology information to characterize resistance profiles and mechanisms of action. It provides a web interface for keyword searches, similarity searches (BLAST and RPSBLAST), mutation-specific searches, and browsing of resistance gene classes. Users can submit information about novel resistance genes through an interface that captures various types of information not commonly available in other databases. ARDB is a valuable resource for researchers studying antibiotic resistance, including microbiologists, clinicians, and the biodefense research community. It is continuously updated and aims to refine its structure, better determine the types of information stored, and improve the user interface. Future efforts will focus on developing new approaches for cataloging and characterizing polymorphisms correlated with resistance and annotating changes to cellular regulatory networks underlying drug tolerance.The Antibiotic Resistance Genes Database (ARDB) is a manually curated database that unifies most publicly available information on antibiotic resistance. It provides detailed annotations for resistance genes, including resistance profiles, mechanisms of action, ontology, COG, and CDD annotations, as well as external links to sequence and protein databases. ARDB supports sequence similarity searches and includes a tool for characterizing common mutations that confer antibiotic resistance. The database contains information for 13,293 genes, 377 types, 257 antibiotics, 632 genomes, 933 species, and 124 genera. ARDB is available at http://ardb.cbcb.umd.edu/. The database was created to address the limitations of existing resources and to facilitate the identification and characterization of antibiotic resistance genes. It provides a centralized compendium of information on antibiotic resistance, facilitates the consistent annotation of resistance information in newly sequenced organisms, and aids in the identification and characterization of new genes. ARDB includes information on 632 complete bacterial genomes, allowing quick search and comparison of resistance profiles. ARDB includes ontology information to characterize resistance profiles and mechanisms of action. It provides a web interface for keyword searches, similarity searches (BLAST and RPSBLAST), mutation-specific searches, and browsing of resistance gene classes. Users can submit information about novel resistance genes through an interface that captures various types of information not commonly available in other databases. ARDB is a valuable resource for researchers studying antibiotic resistance, including microbiologists, clinicians, and the biodefense research community. It is continuously updated and aims to refine its structure, better determine the types of information stored, and improve the user interface. Future efforts will focus on developing new approaches for cataloging and characterizing polymorphisms correlated with resistance and annotating changes to cellular regulatory networks underlying drug tolerance.