The fundamental problem of development is to explain how the progeny of a single cell, the fertilised egg, differentiates to form all the tissues of the body in the right place at the right time. It has only been during the last couple of years that the mechanisms and molecules mediating interactions between cells and tissues have begun to be delineated. This article reviews some of these recent studies. Sequences of topographically defined cellular interactions lead not only to the development of the body but also to the obvious and remarkable inference that the body more or less automatically builds itself.