Simple isostatic models of the development of selected Archaean basins and greenstone belts can be used to constrain models of the Archaean lithosphere. Geophysical, geochemical, and thermal data imply that in southern Africa the late Archaean crust was probably comparable in thickness to the modern continental crust, and the whole lithosphere was perhaps over 80–100 km thick. Two Archaean belts studied may have developed by stretching of the order of 1.3–1.6:1. Isostatic and thermal data and petrological assumptions allow the construction of simple models of the Archaean oceanic lithosphere. Oceans were probably deep: mid-ocean ridges were probably subaqueous and fed by komatiitic liquids. Calc-alkaline volcanism in the Archaean may have been produced as a consequence of the subduction of hydrated komatiitic oceanic crust into a hot, low-viscosity asthensophere.