Daily rhythms of a variety of immunological phenomena and functions are well known, but so far they have largely been neglected. Examples of daily rhythms in the immune system are: circadian differences in susceptibility to bacterial infection and daily variations in the symptoms of diseases such as rheumatoid arthritis or asthma. Therefore, it is very important for clinical diagnosis and pharmacological therapies to elucidate the connections between the circadian clock and the immune system. Although the interplay between the circadian clock and immunological phenomena is obvious, it has not been investigated yet. To date, almost nothing is known about the underlying mechanisms. In this thesis, it first investigated for the first time, whether the immune system – like other peripheral tissues – possesses a circadian clock, which controls immune functions in the different organ tissues. Several criteria, which are evidence for the existence of a circadian clock, are fulfilled in the immune system of mice: (i) The clock genes Per2 and Rev-Erb alpha are rhythmically expressed over 2 days in spleen and lymph nodes. The phases and amplitudes of the oscillations are comparable with other peripheral tissues like lung, liver or kidney. (ii) These circadian oscillations persist for several days in explanted cultures of spleens, lymph notes, peritoneal macrophages and peripheral mononuclear blood cells (PBMCs) of PER2::LUC mice and Per1::luc rats, respectively. Thereby, it was shown that immune tissues and cells contain a circadian clock which is not driven by systemic factors. (iii) This cell-autonomous immune clock potentially regulates immunological functions. Evidence for that are the circadian rhythms in spleen cell number as well as in the cell number of subpopulations like macrophages, B- and T-cells. Another hint is the circadian oscillation of cytokine secretion after stimulation with LPS. Studies with adrenalectomized mice showed that also when cortisol rhythms are absent the oscillation in cell numbers and the rhythm of cytokine secretion of TNF and IL-6 persist. In addition, this work presents evidence for molecular links between the circadian clock network and the immune system signalling networks. To test this hypothesis, the gene expressions in macrophages at two circadian time points were investigated with the help of micro arrays. The data provide indications for a regulation of the toll-like-receptor 4 pathway through the circadian clock. The presented results support the idea of an intrinsic circadian clock work in immune cells, which regulates immune functions. The data on circadian-regulated genes and immune functions contribute to the understanding of how the circadian clock influences the immune system.