The study of circadian clock function in Drosophila relies heavily on the analysis of rhythmic gene expression. Typically, individuals or groups of flies collected during a specific time of the circadian day need to be sacrificed, followed by the extraction of clock gene products. This procedure makes it impossible to analyze molecular rhythms in an individual over time. To measure clock gene expression within the living animal, firefly luciferase can be used as real-time reporter gene. This chapter describes how rhythmic expression of clock or clock-controlled genes can be measured in living adult Drosophila. A survey of all existing clock-related luciferase transgenics is given.