This Review summarizes the state of the art in transition metal-catalyzed alkene hydrocyanation with special emphasis on mechanistic studies. Due to its importance for the DuPont adiponitrile process, most of the literature deals with the nickel-catalyzed hydrocyanation. Ligand electronic and steric effects, as well as the bite angle of chelating ligands, play a dominant role for the catalyst performance. The ligand properties have a major effect on the catalyst stability and on the rate limiting step—the reductive elimination of the products. The DuPont process, comprises three separate steps: a) the hydrocyanation of 1,3-butadiene and other conjugated dienes, b) the isomerization of 2-methyl-3-butenenitrile to 3-pentenenitrile, and c) the hydrocyanation of 3-pentenenitrile and other monoalkenes. Existing knowledge of these steps is summarized and elucidated. For the latter reactions, Lewis acid cocatalysts are imperative, and their influence on the regioselectivity and the catalyst performance will be discussed. Asymmetric alkene hydrocyanation has attracted considerable interest recently and is covered from a mechanistic viewpoint. Finally, a short account is given on other metals applied in alkene hydrocyanation. More detailed mechanistic understanding is still required for the improvement of catalyst performance and to develop this reaction to its full potential in organic synthesis.