This chapter focuses on the similarities and differences between step-growth and chain-growth polymerizations. In nuclear reactors a cascade of fission reactions occurs, which is initiated by the capture of the first neutron. This is also true in chain-growth polymerization. In contrast, chain-growth polymerization occurs by introducing an active growth center into a reservoir of monomer, followed by the addition of many monomers to that center by a chain-type kinetic mechanism. The active center is ultimately inactivated by a termination step. By comparison, a wide variety of unsaturated monomers undergo chain-growth polymerization. A single kind of monomer suffices – more than one yields a copolymer –– and more than one double bond per monomer may result in branching or cross-linking. The (average) degree of polymerization that characterizes the system depends on the frequency of addition steps relative to termination steps. Thus high molecular weight polymer can be produced almost immediately.