Hyperpolarization-activated cation currents termed I (f/h) are observed in many neurons and cardiac cells. Four genes (HCN1-4) encode the channels underlying these currents. New insights into the pathophysiological significance of HCN channels have been gained recently from analyses of mice engineered to be deficient in HCN genes. Lack of individual subunits results in markedly different phenotypes. Disruption of HCN1 impairs motor learning but enhances spatial learning and memory. Deletion of HCN2 results in absence epilepsy, ataxia, and sinus node dysfunction. Mice lacking HCN4 die during embryonic development and develop no sinoatrial node-like action potentials. In the present review, we summarize the physiology and pathophysiology of HCN channel family members based primarily on information from the transgenic mouse models and on data from human patients carrying defects in HCN4 channels.