Recent work on Drosophila limb development has indicated that short-range interactions between distinctly specified populations of cells (compartments) establish organizing centers at compartment boundaries. These organizing centers direct pattern formation and growth in the developing limbs. In the Drosophila wing imaginal disc, there are at least two such organizing centers, located at the anterior/posterior (A/P) and dorsal/ventral (D/V) compartment boundaries. The genetic hierarchies which establish these organizers are starting to be understood, and it also appears that the key mediators of some of the organizers have been identified. Thus Decapentaplegic (Dpp, a secreted signalling molecule of the TGF-B family) is the mediator of the A/P organizer, while Wingless (DWnt-l, a secreted molecule of the Wnt family) is a key mediator of the D/V organizer. In this thesis, several aspects of Wingless function in the wing imaginal disc are examined. Two regulatory mutations, spadeflag (spdfg) and Sternopleural (Sp), that affect Wingless expression in the wing imaginal disc are characterized. The analysis of the mutation spdfg, together with other data, identifies a role of Wingless as a localized mitogen in the developing wing hinge, and also indicates that cells in different regions of the wing disc respond very differently to the Wingless signal. The mutations spdfg and Sp are also among the tools used to examine the position of Wingless in the genetic hierarchy that establishes and mediates the activity of the D/V organizer. These experiments extend the evidence suggesting that Wingless mediates both short-range and long-range effects of the D/V organizer. Wg does so by controlling the expression domains of different target genes, including the acheate-scute genes, Distal-less and vestigial. Finally, the mechanism by which Wingless mediates the activity of the D/V organizer is examined. The results obtained suggest that Wingless functions as a long-range morphogen.