The components specifying the spatial coordinates of the Drosophila embryo are deposited in the egg during oogenesis. Three maternal pathways control the pattern of the embryo along its antero-posterior axis. Genetic and molecular analysis has identified the key-genes in each of these pathways: (1) the bicoid gene encodes an anterior signal in the embryo that directs head and thorax formation via transcriptional activation of anteriorly expressed zygotic genes. (2) A posterior signal, the nanos gene product, antagonizes an inhibitor of abdominal development, hunchback, by translational regulation. (3) A terminal signal controls development at both poles of the embryo. It is probably induced by the somatic follicle cells and transmitted to the embryo via a membrane bound receptor encoded by the gene torso. Other maternal genes function in the localization of these signals or in signal transduction.