Most morphogens are diffusible molecules that regulate cell fate determination during development. The formation of morphogen concentration gradients directs the biological responses of surrounding cells. Graded responses occur as a result of morphogens binding to specific cell surface receptors that subsequently activate intracellular signaling pathways and promote or repress gene expression at specific threshold concentrations. Activation or inactivation of these signaling pathways provides positional information that ultimately determines cell fate decisions, tissue organization, and morphology. Research in model organisms has revealed that morphogenetic fields are involved in many aspects of development. For example, morphogens are required in Drosophila for patterning of the dorso-ventral and anterior-posterior axes, segment patterning, and positional signaling in the leg and wing imaginal discs. Proteins belonging to the Wingless/Wnt, Notch, Hedgehog, and TGF-beta families have been identified as morphogens that direct a number of these processes. Research in higher organisms has demonstrated that homologues of these same signaling molecules regulate vertebrate axis formation, anterior/posterior polarity during limb development, mesoderm patterning, and numerous other processes that establish an organism's basic body structure.