The dorsal-ventral pattern of the Drosophila embryo is established by three sequential signaling pathways. Each pathway transmits spatial information by localizing the activity of an extracellular signal, which acts as a ligand for a broadly distributed transmembrane receptor. The components of the first two pathways are encoded by maternal effect genes, while the third pathway is specified by genes expressed in the zygote. During oogenesis, the oocyte transmits a signal to the surrounding follicle cells by the gurken-torpedo pathway. After fertilization, the initial asymmetry of the egg chamber is used by the spätzle-Toll pathway to generate within the embryo a nuclear gradient of the transcription factor Dorsal, which regulates the regional expression of a set of zygotic genes. On the dorsal side of the embryo, the decapentaplegic-punt/thick veins pathway then establishes patterning of the amnioserosa and dorsal ectoderm. Each pathway uses a distinct strategy to achieve spatial localization of signaling activity.