In reacting flow problems involving large numbers of chemical species and reactions, reduced chemistry modeling is used to approximately describe, with fewer unknowns, the slow chemical evolutions after the rapid but brief initial transients have decayed. Diffusion transport terms, if present, are modified by the reduced chemistry approximations. This article reiterates that the use of unmodified diffusion terms along with reduced chemistry terms is unjustified in general, and discusses the potential impacts of diffusion on the use of "slow manifolds'' to reduce the number of unknowns. In addition, the proper way to handle sidewall reactive-diffusion boundary layers is presented.