Cosmic media are in most cases nonuniform and anisotropic. Examples include stellar envelopes and atmospheres, especially those with strong internal magnetic fields and interstellar gas and dust clouds. Radiation from these objects is polarized. This book is directly concerned with the effects of anisotropy and nonuniformity of cosmic media on the polarization and propagation of stellar and nebular radiation. Appropriate mathematical methods for the calculation of the Stokes parameters are used, and all approximations are analysed in detail. Formulae are given in explicit analytical form convenient for direct comparison with observations. For the sake of illustration, the theory is applied to observations of polarization from a number of astrophysical objects.