A variety of cells including cardiac myocytes and neuronal cells possess inwardly rectifying K+ (Kir) channels through which currents flow more readily in the inward direction than outward. These K+ channels play pivotal roles in maintenance of the resting membrane potential, in regulation of the action potential duration, in receptor-dependent inhibition of cellular excitability, and in the secretion and absorption of K+ ions across cell membrane. Recent molecular biological dissection has shown that the DNAs encoding Kir channels constitute a new family of K+ channels whose subunits contain two putative transmembrane domains and a pore-forming region. So far, more than ten cDNAs of Kir channel subunits have been isolated and classified into four subfamilies: 1) IRK subfamily (IRK1-3/Kir1.1-1.3), 2) GIRK subfamily (GIRK1-4/Kir3.1-3.4), 3) ATP-dependent Kir subfamily (ROMK1/Kir1.1, K(AB)-2/Kir4.1), and 4) ATP-sensitive Kir subfamily (uKATP-1/Kir6.1, BIR/Kir6.2). Xenopus oocytes injected with the cRNAs of IRKs elicit classical Kir channel currents. GIRKs, as heteromultimers, compose the G protein-gated Kir (KG) channels, which are regulated by a variety of Gi/Go-coupled inhibitory neurotransmitter receptors such as m2-mus-carinic, serotonergic (5HT1A), GABAB, somatostatin and opioid (mu, delta, kappa) receptors. ROMK1 and KAB-2 are characterized with a Walker type-A ATP-binding motif in their carboxyl termini, and may be involved in K+ transport in renal epithelial and brain glial cells. uKATP-1 and BIR form with sulfonylurea receptors, the so-called ATP-sensitive K+ channels. Thus, it is a feature of the Kir channel family that each subfamily plays a specific physiological functional role. The (Na+)-activated Kir channels identified electrophysiologically in neurons and cardiac myocytes have not yet been cloned. In this review, we overviewed the current understandings of the features of the molecular structures and functions of the four main subfamilies of Kir channels.