ISOPHOT is one of the instruments on board the Infrared Space Observatory ISO, launched in November 1995 by the European Space Agency. The 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 the use of the instrument team for calibration of the PHT instrument during and after the ISO mission, and as an interactive tool for ISOPHOT data analysis for 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. All the capabilities of input/output, processing and visualization can be reached from window menus, for all the different ISOPHOT sub-systems in all the different levels of data reduction. This applies, starting from digital raw data, coming from the ISO telemetry, to the final level of calibrated images, spectra, multi-filter, multi-aperture photometry. In this article we describe the structure of the package, putting special emphasis on the internal data organization and managing.