The CRISPR-Cas system was originally discovered as an adaptive immune system of bacteria and archaea to protect against viral attack. During this talk, given at Leica Microsystems in Wetzlar, Dr. Lennart Randau, MPI Marburg, presents this simple and fascinating system in detail. Furthermore, he introduces the adaption of the CRISPR-Cas system into a potent molecular biology tool, which is used heavily for genome editing. In addition to its influence on molecular biology, meanwhile the Cas9 nuclease also stimulates microscopy techniques e.g. to fluorescently label genomic loci in living cells.