ISOPHOT is one of the instruments on board the Infrared Space Observatory (ISO), launched in November 1995 by the European Space Agency. ISOPHOT Interactive Analysis, PIA, is a scientific and calibration data analysis tool for ISOPHOT data reduction. The PIA software was designed both as a tool for use by the instrument team for calibration of the PHT instrument during and after the ISO mission, and as an interactive tool for ISOPHOT data analysis by general observers. It has been jointly developed by the ESA Astrophysics Division (responsible for planning, direction, and execution) and scientific institutes forming the ISOPHOT consortium. PIA is entirely based on the Interactive Data Language IDL (IDL92). All the capabilities of input/output, processing, and visualization can be reached from window menus for all the different ISOPHOT sub-systems, in all levels of data reduction from digital raw data, coming from the ISO telemetry, to the final level of calibrated images, spectra, and multi-filter, multi-aperture photometry. In this article, we describe the structure of the package, putting special emphasis on the internal data organization and management.