Nature provides us with an enormous diversity of chemical structures which is an inspiration to chemists and pharmacologists seeking new drug candidates. Circular peptides which have been identified in bacteria, fungi, plants and some mammals but not in humans display exceptional stability and potent bioactivities. They play important roles as defense molecules for their host organisms, providing protection against microorganisms or insects and therefore can be adapted as scaffolds in drug development. Here we try to summarize the most important facts about the origin, biosynthesis and activities of this interesting group of natural products.