A variety of K+ channels have been identified using electrophysiological techniques including the patch-clamp method. The types of channels include inwardly-rectifying, ATP-dependent, Ca(2+)-dependent, voltage-gated, and so on. Some of them have been cloned by expression and/or PCR cloning techniques, which give us their amino-acid sequences and molecular topology in the cell membrane. Immunohistochemical studies have shown the proteins (channels) to be localized to the luminal and/or basolateral membranes of a certain nephron segment. However, the relationships of these proteins with the ionic channels identified functionally must be examined by their reconstitution in cell-free systems (lipid bilayer membrane) and/or their expression in cells lacking native K+ channels. Much more care should also be taken to avoid artifacts due to channel-inducing factors (CHIF). Structure-function studies at the molecular level will advance our knowledge of the renal K+ channels and provide us with a further understanding of the role of the kidney in K+ homeostasis.