The MSIS-83 empirical model of temperature, density and composition is a revision of the original MSIS model (including longitude/UT effects) and is based on temperature, density, and composition data from a comprehensive summary of rocket flights, seven satellites, and five incoherent scatter radars, including data from high solar activity. The model extends the previous description of neutral parameters to the base of the thermosphere in a continuous manner while maintaining the basic structure of the MSIS model at higher altitudes. As the altitude decreases, composition approaches lower atmosphere values, while yearly, and to a lesser extent daily, variations in temperature and density are in reasonable agreement with previous results for the lower thermosphere. An alternate description of magnetic storm variations is provided based on the three hour apindices and an 8- to 10-hour exponential decay in thermospheric density and temperature response after a heating event. Additional coefficients are included for the time independent and magnetic activity terms, including a longitudinally dependent seasonal magnetic activity effect. The molecular oxygen description is based on mass spectrometer and EUV absorption measurements rather than ion chemistry.