The overall objective of the particle environment monitor (PEM) is to provide comprehensive measurements of both local and global energy inputs into the Earth's atmosphere by charged particles and Joule dissipation using a carefully integrated set of instruments. PEM consists of four instruments: the atmospheric X ray imaging spectrometer (AXIS), the high-energy particle spectrometer (HEPS), the medium-energy particle spectrometer (MEPS), and the vector magnetometer (VMAG). AXIS provides global scale images and energy spectra of 3- to 100-keV bremsstrahlung X rays produced by electron precipitation into the atmosphere. HEPS and MEPS provide in situ measurements of precipitating electrons in the energy range from 1 eV to 5 MeV and protons in the energy range from 1 eV to 150 MeV. Particles in this energy range deposit their energy in the atmosphere at altitudes extending from several hundred kilometers down to as low as ~30 km. VMAG Provides the magnetic field direction needed to indicate and interpret the locations and intensities of ionospheric and field-aligned currents as well as providing a reference for the particle measurements. This paper describes each instrument separately and also in the context of the PEM objectives which include the determination of energy deposition and ionization production rates as functions of altitude. Examples of data acquired early in the Upper Atmosphere Research Satellite (UARS) mission are presented. ¿ American Geophysical Union 1993