Since the first discovery of an I_h current in heart pacemaker cells about twenty years ago our knowledge on this ion channel class has increased substantially. Studies from a variety of groups have clearly demonstrated that I_h is not only required for cardiac pacemaking but also is a key regulator of several neuronal functions. The long-sought molecular cloning of the genes underlying I_h has now provided the basis to achieve a deeper understanding of the molecular function and physiological regulation of these channels. In addition, the powerful approaches of mouse genetics will enable us to study the physiological roles of the channels in significantly more detail than it was possible up to now.