A computer code, LOWTRAN 5, has been developed for calculating the transmission and background radiance of the earth's atmosphere at moderate spectral resolution 20 cm-1. The code was derived using a single parameter band model for molecular absorption and includes the effects of continuum absorption, molecular scattering, and aerosol extinction. A choice of atmospheric models and aerosol models is provided to the user for any atmospheric slant path. The LOWTRAN model is described and application of the code is given by examples of atmospheric transmittance and radiance spectra. Comments on the validity of the code are presented.