Voltage-gated ion channels play a critical role in coupling excitation at the neuromuscular junction to activation of contractile elements within a muscle fiber. Abnormal channel function can lead to either muscle paralysis or delayed relaxation. Recent advances in the molecular characterization of these ion channels have provided the tools needed to investigate the relationship between channel mutations and disorders of muscle excitability. This article reviews our current understanding of muscle sodium, calcium, and chloride channels and their role in the pathogenesis of myotonia and periodic paralysis.