PYY3-36 and GLP-1 are two classical gastrointestinal peptides, which are released into the circulation during meals from L-cells of the distal gut; there is compelling evidence that each participates in the control of appetite regulating individual meal sizes in healthy subjects, but also in patients with obesity or diabetes type II. The regulation of human eating habits is, however, highly complex and our understanding of appetite control is far from complete. In many areas our knowledge is rather rudimentary; little is known, to give an example, about the importance of individual signals and their interactions.