Membrane lipids and proteins diffuse laterally in the membrane plane unless constrained by interactions with immobile or slowly moving components. Direct diffusion measurements help characterize the interactions among surface components and can provide information about the mechanism of physiologically interesting processes in membranes. Association of membrane proteins into aggregates may result from changes in intermolecular interactions and will affect the rate of diffusion of the protein. Quantitative measurements of aggregation are therefore becoming important both to aid interpretation of constraints of diffusion and to explore the role of aggregation in physiological events. In this paper, the popular fluorescence photobleaching measurements of diffusion are discussed and their relevance is illustrated with selected examples. Moreover, a variation on the theme of fluorescence correlation spectroscopy is presented as a new approach to measuring the state of aggregation of membrane components on surfaces of living cells.