The gene modification method CRISPR/Cas (CRISPR, clustered regularly interspaced short palindromic repeats; Cas, CRISPR-associated protein) has revolutionized molecular biology within a few years after its first description in 2012. The rapidly increasing number of publications with CRISPR/Cas is only comparable to what was observed in the 1980s after the development of the polymerase chain reaction (PCR) or in the 1990s after the introduction of the green fluorescent protein (GFP). The development of CRISPR/Cas was consequently honored with the Nobel Prize for Chemistry in 202O (Emmanuelle Charpentier & Jennifer Doudna). This chapter provides an insight into the principles and applications of this revolutionary technology in basic research and addresses clinical applications. The use of CRISPR/Cas in plant and animal breeding, which generally involves intervention in the germline, is not the subject of this chapter. Germline modifications with CRISPR/Cas in humans are prohibited worldwide.