Suspension polymerisation of methyl methacrylate was carried out as a model to elaborate on the evolution of particle size average and distribution in the course of polymerisation. Four characteristic intervals in the evolution of particle size were identified as: transition, quasi-steady-state, growth, and identification stages. The effects of stabiliser and initiator concentrations, monomer hold up, reaction temperature, and agitation speed on the characteristic intervals, as well as the kinetics of polymerisation, were examined. The transition stage, which has been totally ignored in the literature, was found to have significant effect on the evolution of particle size. The transition stage is shortened by increasing the rate of polymerisation in the drops (either by increasing initiator concentration or using a higher reaction temperature). Increasing the impeller speed and stabiliser concentration will also lead to a shorter transition period. However, the delayed adsorption of the stabiliser on the surface of drops will prolong the transition stage. It is shown that the occurrence of the quasi-steady state depends on the polymerisation conditions. The quasi-steady state occurs only if the balance between drop break up and coalescence can be maintained. This requires a high rate of drop break up within a period of time during polymerisation (i.e., a low rate of polymerisation in the drops by using a low initiator concentration and reaction temperature, a high agitation speed and a high stabiliser concentration). The mechanisms underlying the growth stage are explained in terms of the overall rates of drop break up and coalescence in the course of polymerisation reactions. It is also shown that the onset of growth stage cannot be defined in terms of a critical conversation or viscosity, and it depends on the polymerisation conditions including mixing. The growth stage occurs if drops are not sufficiently stable against both break up and coalescence. The onset of the growth stage is advanced with a decrease in the rate of drop break up (e.g., decreasing agitation speed and stabiliser concentration). The growth stage can be totally eliminated from a polymerisation process if dispersions with a static steady state can be formed. That requires a high concentration of stabiliser, or a low concentration of monomer, to be used. A population balance model, which included the transition stage and the delayed adsorption of the stabiliser, was developed that is capable of predicting the evolution of drop size in the suspension polymerisation.