The immune system function oscillates with a 24-hour period driving circadian rhythmicity of immune responses. A circadian timing system comprising central and peripheral oscillators entrains body rhythmicity of physiology and behavior to environmental cues by means of humoral signals and autonomic neural outputs. In every single cell an oscillator goes ticking through a molecular clock operated by transcriptional/translational feedback loops driven by the rhythmic expression of circadian genes. This clock gene machinery steers daily oscillations in the regulation of immune cell activity, driving the periodicity in immune system function. The transcriptional networks that regulate temporal variation in gene expression in immunocompetent cells and tissues respond to diverse physiological clues, addressing well-timed adjustments of transcription and translation processes. Nuclear receptors comprise a unique class of transcriptional regulators that are capable of gauging hormones, metabolites, endobiotics and xenobiotics, linking ligand sensing to transcriptional responses in various cell types through switching between coactivator and corepressor recruitment. The expression of coregulators is highly responsive to physiological signals, and plays an important role in the control of rhythmic patterns of gene expression, optimizing the switch between nycthemeral patterns, and synchronizing circadian rhythmicity with changing physiological demands across the light-dark cycle. The nuclear receptors and transcription factors expressed in the immune components contribute to the cross-talk between the circadian timing system, the clock gene machinery and the immune system, influencing transcriptional activities and directing cell-type specific gene expression programs linked to innate and adaptive immune responses.