Chloride channels are ubiquitous proteins found in invetebrates to man. C1-is one of the most abundant biological anions and accounts for a measurable fraction of the electrical conductance of many biological membranes. Physiologically this contributes to cellular processes, including pH regulation, volume regulation, generation of the resting membrane potential, and regulation of membrane excitability. The unitary conductance of voltage-dependent C1-channels is as diverse as the number of different types of C1-channels described ranging from 5-450 pS. C1channels are highly anion selective passing at least ten anionic species, including all of the halides. C1-channels are blocked by various agents, including aromatic acids, inorganic cations, and protons. Maintaining high resting conductance and normal excitability, regulating cell volume, and modulating hormone action are some examples of the functions of C1-channels. Despite the large amount of data accumulated on voltage-dependent C1-channels, identifying subsets within this class of channels with coherent biophysical features that subserve each specific function is still not possible. At present, the molecular structure for every type of functional Cl-channels has not been determined, but future identification of cloned Cl-channel structures should provide a clearer understanding of the functional properties of background C1-channels.