Morphogens provide the positional information that is used to form patterns of distinct cell types in developing tissues. But can morphogen gradients alone determine these precise patterns or do they establish a relatively coarse pattern that is refined later? A recent study of the Bicoid gradient in fly embryos supports the first theory. However, for the Decapentaplegic (Dpp)morphogen gradient in developing Drosophila wing discs, Bollenbach and colleagues now suggest that the second scenario is more appropriate (see p. 1137). These researchers have developed a theoretical description of Dpp gradient formation and have also measured Dpp gradients, Dpp signalling activity and the activation of the Dpp target gene spalt (which controls vein pattern)in developing wing discs. These approaches indicate that the Dpp gradient positions the Spalt boundary to an accuracy of about three cells in the discs. Because the final vein pattern is more precise than this, the researchers suggest that downstream events must refine the positional information provided by the Dpp gradient.