Polarization phenomena in the optical absorption and emission of metallic, semiconducting or composite nanowires are considered theoretically. Most nanowire-based structures are characterized by a dramatic difference in dielectric constant ε between the nanowire material and environment. Due to image forces caused by such ε mismatch in nanowire structures, coefficients of their absorption and emission become essentially different for light polarized parallel or perpendicular to the nanowire axis. As a result, the intensity and spectra of absorption, luminescence, luminescence excitation, and photoconductivity in nanowires or arrays of parallel nanowires are strongly polarization-sensitive. In light-emitting nanowire core-shell structures, the re-distribution of a.c. electric field caused by the image forces may result in essential enhancing of core luminescence in frequency regions corresponding to luminescence from the semiconducting core or when the frequency of optical excitation coincides to the frequency of the plasmon resonance in the metallic shell. Random nanowire arrays acquire some properties typical for nematic liquid crystals. In such arrays, the effect described above may result in "polarization memory", where polarization of luminescence is determined by the polarization of the exciting light.