One mechanism by which spatial patterns of cell differentiation could be specified during embryonic development and regeneration is based on positional information. Cells acquire a positional value with respect to boundaries and then interpret this in terms of a programme determined by their genetic constitution and developmental history. The signals and the molecular basis of such a system have both been rather well conserved. Recent work has shown that cells can respond to quite small differences in the concentrations of molecules whose concentration could provide positional information.