Topics in finite temperature quantum field theory

In this dissertation, we investigate different aspects of certain field theoretic models at zero and nonzero temperature. In Chern-Simons (CS) theories, we compute radiative corrections to the CS mass term at two loops in (2 + 1)-dimensional quantum electrodynamics at T \u3e 0. In contrast to a well known result at zero temperature, the CS mass term does receive nonzero contributions beyond one loop at nonzero temperature. We investigate the phenomenon of fermion number fractionization at finite temperature in a variety of models of increasing complexity in (1 + 1), (2 + 1) and (3 + 1)-dimensions. Using the derivative expansion technique we find that, contrary to well known theorems at zero temperature, the induced fermion number is generically nontopological, i.e, is sensitive to the detailed profile of the background field, and is not a sharp observable, i.e, has nonvanishing rms fluctuations.