A biosystem, or biological system, is a group of molecules that interact in a biological system. One type of biosystem is a biological pathway, which can consist of interacting genes, proteins, and small molecules. Another type of biosystem is a disease, which can involve components such as genes, biomarkers, and drugs.

A number of databases, such as KEGG, BioCyc, and Reactome provide diagrams showing the components and products of biological pathways along with corresponding annotations and links to literature. The NCBI BioSystems Database was developed as a complementary project to (1) serve as a centralized repository of data; (2) connect the biosystem records with associated literature, molecular, and chemical data throughout the Entrez system; and (3) facilitate computation on biosystems data.

The NCBI BioSystems record for arachidonic acid metabolism, for example, displays the name and description of the biosystem along with a thumbnail image of the pathway diagram that links to the full size illustration on the source database's web site. In addition, the BioSystems record lists and categorizes the genes, proteins, and small molecules involved in the biological system, along with related biosystems and citations, and allows instant retrieval of the those data sets through a wide range of Links. Integrating the data in this way makes it possible to search across all the pathways to answer broad questions such as the "how to" examples shown below.

The NCBI BioSystems Database currently contains biological pathways from three source databases: KEGG, the EcoCyc subset of BioCyc, and the human subset of Reactome. It is also designed to accommodate other types of biosystems such as diseases as data about them become available. Through these collaborations, the BioSystems database facilitates access to, and provides the ability to compute on, a wide range of biosystems data. Detailed diagrams and annotations for individual biosystems are then available on the web sites of the source databases.