The mammalian molecular clock comprises a complex network of transcriptional programs that integrates environmental signals with physiological pathways in a tissue-specific manner. Emerging technologies are extending knowledge of basic clock features by uncovering their underlying molecular mechanisms, thus setting the stage for a 'systems' view of the molecular clock. Here we discuss how recent data from genome-wide genetic and epigenetic studies have informed the understanding of clock function. In addition to its importance in human physiology and disease, the clock mechanism provides an ideal model to assess general principles of dynamic transcription regulation in vivo.