The GAL4/upstream activating sequence (UAS) system is one of the most powerful tools for targeted gene expression. It is based on the properties of the yeast GAL4 transcription factor which activates transcription of its target genes by binding to UAS cis -regulatory sites. In Drosophila , the two components are carried in separate lines allowing for numerous combinatorial possibilities. The driver lines provide tissue-specific GAL4 expression and the responder lines carry the coding sequence for the gene of interest under the control of UAS sites. In this chapter, the basic GAL4/UAS system and its extensions, namely those allowing precise temporal control and reversible expression, are described. In addition, a list of GAL4 and UAS lines and schematic maps of GAL4 and UAS vectors useful in the study of Hedgehog (Hh) signaling is given. Finally, uses of the GAL4/UAS system to resolve some of the questions addressed in the study of the Hh pathway are presented.