The metameric organization of the Drosophila embryo is generated in the first 5 h after fertilization. An initially rather simple pattern provides the foundation for subsequent development and diversification of the segmented part of the body. Many of the genes that control the formation of this pattern have been identified and at least twenty have been cloned. By combining the techniques of genetics, molecular biology and experimental embryology, it is becoming possible to unravel the role played by each of these genes. The repeating segment pattern is defined by the persistent expression of engrailed and of other genes of the 'segment polarity' class. The establishment of this pattern is directed by a transient molecular prepattern that is generated in the blastoderm by the activity of the 'pair-rule' genes. Maternal determinants at the poles of the egg coordinate this prepattern and define the anteroposterior sequence of pattern elements. The primary effect of these determinants is not known, but genes required for their production have been identified and the product of one of these, bicoid is known to be localized at the anterior of the egg. One early consequence of their activity is to define domains along the A-P axis within which a series of 'cardinal' genes are transcribed. The activity of the cardinal genes is required both to coordinate the process of segmentation and to define the early domains of homeotic gene expression. Further interactions between the homeotic genes and other classes of segmentation genes refine the initial establishment of segment identities.