Luminescence reporters have been used successfully in studies of circadian rhythms. Real-time measurements of circadian variations in gene expression were made in living cells, cultured tissues, and whole organisms. Because this technique is relatively easy and continuous noninvasive measurement from tissue cultures allows for a drastic reduction in the number of experimental animals, we believe this method will become a common technique for studying circadian rhythms. Using a multichannel recording apparatus, it may also become a powerful tool for the discovery of new drugs. In the past, measurements were done using hand-made apparatuses or by modifying commercially available equipment. We, along with other investigators, have developed user-friendly equipment for performing circadian rhythms experiments, and these systems are now available commercially. This article describes the use of luminescence reporters in circadian research and provides detailed methods used in these experiments. One of our goals in this article is to reduce experimental variability in different laboratories by proposing standard protocols.