Reporter gene assays are versatile and sensitive methods of assaying numerous targets in high-throughput drug-screening programs. A variety of reporter genes allow users a choice of signal that can be tailored to the required sensitivity, the available detection apparatus, the cellular system employed, and the required compatibility with multiplexed assays. Promoters used to drive reporter gene expression can be activated either by a broad range of biochemical pathways or by the selective activation of individual targets. In this chapter, we will introduce some of the considerations behind the choice of reporter gene assays and describe the methods that we have used to establish 96-well format luciferase and aequorin assays for the screening of ligands for G protein-coupled receptors.