AbstractCircadian rhythms are responsible for 24‐hour oscillations in diverse biological processes. While the central genes governing circadian pacemaker rhythmicity have largely been identified, clock‐controlled output molecules responsible for regulating rhythmic behaviors remain largely unknown. Two recent reports from McDonald and Rosbash1 and Claridge‐Chang et al.2 address this issue. By identifying a large number of genes whose mRNA levels show circadian oscillations, the reports provide important new information on the biology of circadian rhythm. In addition, the reports illustrate both the power and limitations of microarray‐based methods for profiling mRNA expression on a genomic scale. BioEssays 24:494–498, 2002. © 2002 Wiley Periodicals, Inc.