In recent years metathesis polymerization has made an increasingly significant impact on the advanced materials arena. These developments may largely be attributed to the introduction of a new generation of metathesis initiator in which the active site is incorporated within a relatively robust transition metal complex, shielded from outside interference by a protective array of ancillary ligands. One of the principal advantages of these systems is that they do not require a co-catalyst for activation which in the past has proved problematical in defining the active site and for the tolerance of heteroatom functionalities. Moreover, their activity and selectivity can be closely controlled by appropriate choice of the ancillary groups, allowing a high degree of control over the resultant polymer microstructure. In many cases the process is 'living' and thereby may be exploited to synthesize polymers of narrow molecular weight distributions and block co-polymers. Here we shall review some of the key developments that have taken place since the introduction of well-defined transition metal initiators, the types of material which are accessible, the control over microstructure now possible and in particular the advantages the latest generation initiators hold for the design and synthesis of new materials. The literature on metathesis polymerization is extensive and much of the work up until the mid-1980s has been comprehensively reviewed.([1])] This review will focus on developments that have taken place since the introduction of well-defined initiators in the mid-1980s, aspects of which have also been out-lined elsewhere.[(2)]