Synthetic organic reactions through C-CN activation by transition metal catalysis are reviewed. C-CN bond activation by metal complexes proceeds mainly via two pathways; oxidative addition and C-CN cleavage accompanied by silylisonitrile formation. Both the elemental reactions have been successfully applied to the catalytic reactions, including hydrodecyanation of nitriles, cross-coupling using nitriles as electrophiles, cyanation of aryl halides and arenes using organic nitriles as cyanating agents, and carbocyanation of unsaturated compounds.