A theory for polymer molecular weight distribution (MWD) in atom transfer radical polymerization (ATRP) has been derived by using analogy to a series of continuous stirred tank reactors (CSTRs). This approach relates one activation cycle in ATRP to one reactor in CSTRs. The derived MWD expression includes effect of radical termination and allows detailed investigation on the factors that determine the polymer distribution, namely the level of control and livingness. The level of control means the average number of activation cycles experienced by individual chains. The degree of livingness is quantified by the fraction of terminated chains. It was found that the effect of livingness and control on MWD is complex. Large number of activation cycles does not guarantee a narrow distribution. There exists an optimum value of activation cycles to achieve the lowest polydispersity by balancing the control (achieved by increasing activation cycles) with the loss of livingness (also caused by increasing activation cycles). The distribution for ATRP is in-between Poisson and Flory distributions, determined by the level of control and livingness. In general, having high degree of livingness and maintaining control are both necessary conditions to obtain narrow MWD. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 639-651