We review here the theoretical methods that are currently in use for studying organometallic reactions. Selected examples are taken from the field of homogeneous catalysis, with particular emphasis on olefin polymerization processes. These are chosen to illustrate the importance of the choice of the model; the various techniques that are used to characterize the transition state; the influence of electron correlation to account for weak interactions; the use of QM/MM methods to model the steric effects of bulky ligands. Special attention is paid to dynamic simulations that have recently been introduced in such studies, and to the inclusion of solvent effects.