The Mass-Spectrometer-Incoherent-Scatter (MSIS) model describes the neutral temperature and densities in the upper atmosphere (above about 100 km). MSIS-86 constitutes the upper part of the COSPAR International Reference Atmosphere (CIRA) 1986. The MSIS model is based on the extensive data compilation and analysis work of A. E. Hedin and his colleagues. Data sources include measurements from several rockets, satellites (OGO 6, San Marco 3, AEROS-A, AE-C, AE-D, AE-E, ESRO 4, and DE 2), and incoherent scatter radars (Millstone Hill, St. Santin, Arecibo, Jicamarca, and Malvern). The model expects as input year, day of year, Universal Time, altitude, geodetic latitude and longitude, local apparent solar time, solar F10.7 flux (for previous day and three-month average), and magnetic Ap index (daily or Ap history for the last 59 hours). For these conditions the following output parameters are calculated: number density of He, O, N2, O2, Ar, H, and N, total mass density; neutral temperature and exospheric temperature. For diagnostic purposes the source code is equipped with 23 flags to turn on/off particular variations. Hedin (1988) compared all three MSIS models with each other and with the Jacchia 1970 and 1977 models.