This chapter examines how the degree of polymerization and its distribution vary with the progress of the polymerization reaction, with the latter defined both in terms of composition and kinetics. It considers these topics for simple reaction mixtures, that is, those in which the proportions of reactants agree exactly with the stoichiometry of the reactions. As the name implies, step-growth polymers are formed through a series of steps, and high molecular weight materials result from a large number of steps. Although our interest lies in high molecular weight, long-chain molecules, a crucial premise of this chapter is that these molecules can be effectively discussed in terms of the individual steps that lead to the formation of the polymer. It is also possible that the transport of reactants will be affected by the increasing viscosity of the polymerization medium, which is a very complicated issue.