A kinetic model incorporating effects of diffusion-controlled reactions on atom-transfer radical polymerization (ATRP) was developed. The reactions considered to be diffusion-controlled are monomer propagation, bimolecular radical termination, chain transfer between propagating radical and catalyst, and transfer to small molecules. Model predictions indicate that a diffusion-controlled propagation reduces the "living" behavior of the system, whereas a diffusion-controlled termination enhances its livingness. When diffusion-controlled transfer between chains and catalyst is considered the same in the forward and backward directions, the livingness of the system is enhanced, but if one of them is kept unchanged while the other is increased, the livingness of the system is reduced. When diffusion-controlled termination is important, our simulations show that the overall effect of diffusion-controlled phenomena in A TRP is to enhance the livingness of the system. Experimental data from the literature for styrene, methyl methacrylate, and methyl acrylate ATRP homopolymerizations validate the kinetic model.