The spinal muscular atrophies are common causes of muscle weakness and atrophy in children. These disorders, previously referred to as Werdnig-Hoffman disease and Kugelberg-Welander disease, are now much better diagnosed and understood, particularly after the identification of the survival motor neuron gene and their common linkage to the long arm of chromosome 5. This chapter discusses the clinical manifestations, genetics, diagnosis, and management of the spinal muscular atrophies.