Activation of pacemaker channels underlie the spontaneous diastolic depolarization of sinoatrial node cells in the heart. Four similar genes encoding these hyperpolarization-activated, cyclic nucleotide-gated channels were recently cloned and subsequently named HCN1-4. Here we review the physiological role of HCN channels and recent findings regarding mechanisms of channel gating. Like all other voltage-gated channels, site-directed mutagenesis analysis indicates that the highly charged S4 transmembrane domain is the voltage sensor. However, unlike most other channels channel, opening occurs in response to membrane hyperpolarization rather than depolarization.