Submillimeter Array Technical Memo #152 version 9.2, describing the underlying physics and usage of am, a software tool for radiative transfer computations at millimeter and submillimeter wavelengths. The primary application areas for am are in radio astronomy and terrestrial atmospheric radiometry. The am program and this memo are made available under the following license: This computer program containing an atmospheric propagation model for the submillimeter band is a work of the United States and may be used freely, with attribution and credit to the Smithsonian Astrophysical Observatory. The program is intended for educational, scholarly or research purposes. In connection with any commercial use of the program, the user should disclose clearly and conspicuously all of the information contained in the first sentence of this notice.