The investigation of circadian clock function in Drosophila has progressed from the identification of clock genes to the analysis of timing mechanisms in the cells and tissues where these genes are expressed. As the biological context for investigating circadian clock systems is expanded, new features of molecular timing mechanisms are becoming apparent. Examples come first from studies on peripheral clocks, which perform local, tissue-specific functions as well as global functions that relate to the control of individual behavior, and second from the evaluation of social influences on circadian rhythms. The identification of inter-organismal components of the circadian system in Drosophila suggests new perspectives as the progression continues from the systems level to the social level and onwards to the level of ecosystems.