The propagation mechanism of free-radical copolymerisation is critically reviewed and it is concluded that the 'explicit' or complete penultimate model should be regarded as the base model for the majority of copolymerisation reactions. This conclusion is based on the weight of evidence from the cumulative findings of a diverse range of studies, viz. the determination of average propagation rate coefficients in co and terpolymerisation systems, moderated copolymerisation studies, solvent effects studies, ESR studies of model-radicals, and both experimental and theoretical studies of gamma-substituent effects in small-radical addition reactions. The consequences of this finding for the understanding of copolymerisation are wide-ranging. In particular, it implies that the widely published terminal-model reactivity ratios should only be regarded as adjustable parameters that do not adequately reflect the elemental processes in the copolymerisation mechanism. In addition, the studies reviewed in this paper indicate that a number of factors, including polar interactions, radical stabilisation effects, direct interactions, and entropic effects all are responsible for the penultimate unit effect in free-radical copolymerisation.