In this chapter, reported syntheses of vanadium–alkylidene complexes and some of their reactions and applications as catalysts for ring-opening metathesis polymerisation (ROMP) of cyclic olefins are summarised. (Imido)vanadium(v)-alkylidene complexes of the type V(CHSiMe3)(NR)(X)(L)n [R=adamantyl, aryl; X=phenoxide, alkoxide etc.; L=PMe3 etc.; n=1,2] exhibit moderate to remarkable catalytic activities for the ROMP of norbornene and the derivatives. The ligand modification in V(CHSiMe3)(N-2,6-Cl2C6H3)[OC(CF3)3](PMe3)2 enables the cis-specific ROMP; the activity increased at high temperature (80 °C) with high specificity (>98%) upon addition of PMe3. The activities were affected by the ligand substituents, and V(CHSiMe3)(N-2,6-Cl2C6H3)(OC6Cl5)(PMe3)2 catalyses ROMP of cycloheptene and cis-cyclooctene; the ROMP of cis-cyclooctene proceeded in a living manner even at 80 °C.