Patches (lateral heterogeneities) of cell surface membrane proteins and lipids have been imaged by a number of different microscopy techniques. This patchiness has been taken as evidence for the organization of membranes into domains whose composition differs from the average for the entire membrane. However, the mechanism and specificity of patch formation are not understood. Here we show how vesicle traffic to and from a cell surface membrane can create patches of molecules of the size observed experimentally. Our computer model takes into account lateral diffusion, barriers to lateral diffusion, and vesicle traffic to and from the plasma membrane. Neither barriers nor vesicle traffic alone create and maintain patches. Only the combination of these produces a dynamic but persistent patchiness of membrane proteins and lipids.