Volume 5 of Biomembranes covers an important group of membrane proteins, the ATPases. The P-type ATPases couple the hydrolysis of ATP to the movement of ions across a membrane and are characterized by the formation of a phosphoyrlated intermediate. Included are the plasma membrane and muscle sarcoplasmic reticulum Ca²⁺ -ATPases, the (Na⁺ -K⁺) -ATPase, the gastric (H⁺ -K⁺) -ATPase, the plasma membrane H⁺ -ATPase of fungi and plants, the Mg2+ - transport ATPase, the Salmonella typhimurium, and the K⁺ -ATPase of Escherichia coli, KdpB. The other important classes of ATPase in eukaryotic systems are the vacuolar H⁺ -ATPases and the F0F1 ATP synthase, and, in bacteria, the anion-translocating ATPases, responsible for resistance to arsenicals and antimonials, and the (Na⁺ -Mg²⁺) -ATPase of Acholeplasma. Finally, eukaryotic systems contain a variety of ectonucleotidases important, for example, in hydrolysis of extracellular ATP released as a cotransmitter from cholinergic and adrenergic nerve terminals. Volume 5 of Biomembranes explores structure-function relationships for these mebrane-bound ATPases.