Selective cleavages of carbon-carbon bonds catalyzed by transition-metal complexes have been shown to be increasingly versatile tools for organic synthesis allowing for complementary synthetic strategies. Numerous examples of transition-metal catalyzed C-C bond activations of three- and four-membered ring systems have been reported. These strained rings have been shown to engage in a variety of new ring-opening rearrangements and cycloaddition reactions leading to valuable structures. Besides strain-driven transformations, other facilitating strategies to enforce the C-C bond activation of unstrained molecules have been developed as well. While the variety of different transformations is less abundant, they concentrate on chelation-assisted reactions using appropriate permanent or transient directing groups. In particular, the cleavage and subsequent functionalization of the C-CN bonds and decarbonylation processes operating by an excision of carbon monoxide from ketone derivatives have witnessed a large progress.