Systems Biology Pathway Exchange (SBPAX) is an ontology and data format designed to store and organize systems biology knowledge data related to biological pathways, quantitative modeling, and the relationships between these two. Originally developed as an extension to BioPAX Level 2, it is now being developed as an extension to BioPAX Level 3 (the current BioPAX version) and proposed as an addition to the upcoming BioPAX Level 4. SBPAX enables broad interoperation between BioPAX and other systems biology modeling formats by creating hierarchies into which data, terms, and data objects from other formats can be inserted. While technically, SBPAX can incorporate any controlled vocabulary, some are better suited for this purpose than others. The preferred choice is the Systems Biology Ontology (SBO). Other suitable sources of terms would be the Systems Biology Markup Language (SBML), the Virtual Cell Markup Language (VCML), CellML, and MathML.