Potassium channels are the most widely distributed type of ion channel and are found in virtually all living organisms. They form potassium-selective pores that span cell membranes, and are found in most cell types and control a wide variety of cell functions. This book presents current research in the study of potassium channels, including potassium channel activity in the regulation of vascular tone in physiological and pathophysiological conditions; the contribution of lymphocyte potassium channels in the perinatal regulation of the immune response in both mother and the newborn; slow potassium channel dysfunction in amyotrophic lateral sclerosis and plant mitochondrial potassium channels.