Atmospheric models have been calculated for M dwarf stars found in post- common envelope binary systems and extra-solar giant planets (EGPs) which have very small orbital separations. In such systems, the radiation eld from the hotter companion effects the atmosphere of the much cooler M dwarf or planet. The PHOENIX model atmosphere code has been adapted to explicitly include the extrinsic radiation in the solution of either the plane parallel or spherically symmetric radia- tive transfer equation and in the calculation of the temperature structure. The updated PHOENIX code also employs a new modied Unsold-Lucy temperature cor- rection procedure that is stable and ensures that energy conservation is satised even in the presence of strong extrinsic ux.|Irradiated planets located at various orbital separations from either a dM5 or a G2 primary star have been modeled for two extreme cases: one where dust clouds form and remain suspended in the atmosphere, and another where dust clouds form but completely settle out of the atmosphere. The atmospheric structure and emer- gent spectrum strongly depend on the presence or absence of dust clouds. It has also been demonstrated that neutral sodium is not in local thermodynamic equilibrium (LTE) in the outer atmosphere of irradiated EGPs.|Pre-cataclysmic variables (pre-CVs) containing a hot white dwarf (WD) and cool M dwarf have also been modeled. These models demonstrate that dramatic changes can occur in the atmosphere of an M dwarf secondary due to the incident ux from a WD primary. A large temperature inversion, similar in appearance to a chromospheric transition region, forms in the upper atmosphere of the secondary. Also, the eects of irradiation have been shown to vary across the surface of the secondary leading to distinct spectra at dierent orbital phases and inclinations.