Cyclin-dependent kinase-5 (CDK5) is a serine-threonine protein kinase that has been implicated in a number of physiological processes in nerve and muscle cells, including neurogenesis, neuritic outgrowth, axonal transport of membrane-bound organelles and myogenesis. CDK5 has also been shown to phosphorylate the important cytoskeletal proteins, neurofilament and tau, both in vitro and in cells. The latter has prompted study into the potential role of CDK5 in the hyperphosphorylation of these proteins as part of the neuropathology seen in amyotrophic lateral sclerosis (ALS) and other neurodegenerative diseases. More recently, increasing evidence has suggested a role for CDK5 in cellular apoptosis. Apoptosis has been implicated as the final common pathway of cell death in a number of neurodegenerative diseases including ALS. This article sets out to review the physiological and pathological roles ascribed to CDK5 and the possible relevance thereof to the pathogenesis of ALS.