Two principal approaches are identified in the modeling of chemical transformations of SOx and NOx in the polluted atmosphere. The fundamental approach involves simulation of the detailed chemical kinetics of the SOx-NOx-HC system; in the empirical approach, relatively simple parameterizations of the bulk rates of conversion of precursors to secondary products are sought in terms of environmental factors which, based on laboratory and field measurements appear to control these rates. In this paper, the principal features of both approaches are described in the form of an overview, with examples of the main results. An assessment is made of the current state of development of both methods, particularly within the context of their applicability in regional transport-transformation models. A brief overview is also presented of the controversial question of linearity of the transformation module with respect to the precursor emissions.