The past 5 years has witnessed an advance in our understanding of alpha1G (Ca_v3.1), alpha1H (Ca_v3.2), and alpha1l (Ca_v3.3), the pore-forming subunits of T-type or low-voltage-activated calcium channels (LVAs). LVAs differ in their localization and molecular, biophysical, and biochemical properties, but all conduct a transient calcium current in a variety of cells. T-type currents mediate a number of physiological functions in developing and mature cells, and are implicated in neural and cardiovascular diseases. Hampered by a lack of selective antagonists, characterization of T-type channels has come from recombinant channel studies and use of pharmacological and electrophysiological methods to isolate endogenous T-type currents. The surprising heterogeneity in T-type currents likely results from differences in LVA molecular composition, temporal and spatial localization, and association with modulatory molecules. A fundamental knowledge of LVA biochemical properties, including the molecular composition of endogenous LVAs and spatial and temporal characterization of protein expression, is necessary to elucidate mechanisms for regulation of expression and function in normal and diseased cells.