One of the primary goals of the particle environment monitor (PEM) on the Upper Atmosphere Research Satellite is the determination of the solar and magnetospheric particle energy inputs to the upper and middle atmosphere. This is accomplished by measuring the incident particle spectra and computing energy deposition and ionization rates as functions of altitude. The particle spectral measurements are made by detectors covering the energy range 5 eV to 5 MeV for electrons and 5 eV to 150 MeV for protons. An X ray camera provides global-scale images and energy spectra of ~3 to 100 keV bremsstrahlung X rays produced by electrons incident on the atmosphere. Thus PEM measures particles which lose their energy in an atmospheric region extending from the thermosphere down to ~50 km, with the altitude of associated bremsstrahlung X rays reaching ~25-km levels. PEM validation documented in this report has involved an assessment of the data quality of each PEM component instrument, intercalibration of individual instruments, and validation of the computations of the rates of electron energy deposition into the atmosphere. The energy deposition rate calculations are validated against existing energy deposition codes over the complete energy range of PEM measurements. ¿ American Geophysical Union 1996