Graded signals are an important component of current models of pattern formation. Typically, a group of cells produces a signal that decays as it spreads through neighboring tissue. By contrast with endocrine signals, which spread systemically, patterning signals or morphogens have a restricted zone of influence, an area classically known as a field. The widely accepted model is that graded distribution of such signals allow cells to measure their position relative to the source. Although it provides a framework for understanding pattern formation, the concept of the morphogen raises many mechanistic issues. For example, how the distribution of a morphogen is established and maintained remains an outstanding issue. There is no doubt that signals are transported over distances of tens of cell diameters and that stable gradients do form. The question of how this is achieved has aroused the interest of many cell biologically minded developmental biologists.