Utilization of the near-millimeter-wave band is limited by atmospheric effects on propagation. Although this paper is restricted to clear-air effects, it is pointed out that these phenomena, which are molecular in origin, are also present to a significant degree in adverse weather. The paper surveys theoretical and observational knowledge concerning absorption, emission, refraction, and turbulence effects in clear air and in the spectral range 90 to 1000 GHz. Modeling practices are also reviewed.