The Fluid Mosaic model of biological membranes, formulated over 20 years ago, implies that receptors float freely within the plane of the membrane. This is a critical exposition of the theory behind and the evidence for the mobile receptor hypothesis. It treats the direct evidence for the mobile receptor hypothesis with particular respect to tyrosine kinase receptors such as those for insulin and epidermal growth factor, and the GTP-binding protein activating V1- and V2-type receptors for vasopressin.