We examine the evolution of the galaxy luminosity function (LF) using a sample of over 2000 galaxies, with 0.12 < z < 0.55 and 17.0 < Rc < 21.5, drawn from the Canadian Network for Observational Cosmology Field Galaxy Redshift Survey (CNOC2), at present the largest such sample at intermediate redshifts. We use UBVRcIc photometry to classify our galaxies into early, intermediate, and late types, for which we compute luminosity functions in the rest-frame B, Rc, and U bandpasses. In particular, we adopt a parameterization of LF evolution including luminosity and number density evolution, and take care to quantify correlations among our LF evolution parameters. Our principal result is a clear quantitative separation of luminosity and density evolution for different galaxy populations, and the finding that the character of the LF evolution is strongly dependent on galaxy type. Specifically, the early- and intermediate-type LF's show primarily brightening at higher redshifts and only modest density evolution, while the late-type LF is best fit by strong number density increases at higher z, with little luminosity evolution. We also carefully measure and account for sample selection effects as functions of galaxy magnitude and color, and show that our results are not significantly affected by potential systematic effects, such as surface brightness selection, photometric errors, or redshift incompleteness. (abstract abridged)

57 pages, including 21 figures; ApJ, in press. Also available at http://manaslu.astro.utoronto.ca/~carlberg/cnoc/index.html