To contribute some brief theoretical context to the main observational questions posed in this session, this presentation will review how trends in galactic star formation rates, as a function of cosmic time and of stellar mass, are to a great extent determined by fundamental physical and cosmological parameters. After outlining the important analytic arguments behind these links, we illustrate how these trends emerge in numerical models. By then using these models to generate mock samples, and running these through the same analysis that would be applied to real data, we demonstrate how our observational conclusions depend on sample selection limits and on the working assumptions adopted for star formation histories.