Cardiac pacemaking is controlled by a mixed Na⁺/K⁺ current named I_f, which is activated by hyperpolarized membrane potentials. Recently, a family of hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channels has been cloned. The members of this family exhibit the general features of I_f channels. This review describes the molecular diversity of the HCN channel family and the structural determinants of channel function including activation by voltage, modulation by cylic nucleotides and ion permeation. The relationships beween cloned HCN channel types and native cardiac I_f currents are explored.