This report describes the capabilities and use of the JUPITER API - the Joint Universal Parameter IdenTification and Evaluation of Reliability Application Programming Interface. This API provides resources for programming applications (computer programs) designed to analyze process models. Process models simulate, for example, physical, chemical, and(or) biological processes of a site or experiment of interest. The analyses involved could include sensitivity analysis, data needs assessment, calibration, uncertainty evaluation, optimization, and other types of applications that require similar capabilities.