The muscle Cl- channel, ClC-1, is a member of the ClC family of voltage-gated Cl- channels. Mutations in CLCN1, the gene encoding this channel, cause two forms of inherited human muscle disorders: recessive generalized myotonia congenita (Becker) and dominant myotonia (Thomsen). The functional characterization of these naturally occurring mutations not only allowed a better understanding of the pathophysiology of myotonia, it also provided important insights into the structure and function of the entire ClC channel family. This review describes recent experiments using a combination of cellular electrophysiology, molecular genetics, and recombinant DNA technology to study the molecular basis of ion permeation and selection in ClC-type chloride channels.