The circadian clock network of D. melanogaster consists of about 150 clock neurons which are located in the lateral and dorsal brain. Communication between these neurons as well as between clock neurons and putative clock output sites is thought to be mainly achieved by neuropeptides. The most important “clock” peptide is the pigment dispersing factor (PDF), which is highly conserved among different insect groups. Disturbance of the PDF circuit leads to a severe impairment of normal rhythmic behavior in Drosophila. The focus of my PhD thesis lies in the investigation and identification of other putative “clock” neuropeptides. Possible candidates are the Neuropeptide F (NPF), short Neuropeptide F (sNPF) and the Ion Transport Peptide (ITP). After manipulating peptide circuits – which is achieved mainly genetically – I investigate the impact on the clock function both on the behavioral and on the neuronal level. Methods I employ are immunocytochemistry, behavioral assays, live imaging and molecular genetics.