Hitherto, various methods have been developed for targeted genome editing, including Zinc Finger nucleases (ZFNs), transcription activating effector nucleases (TALEN), and CRISPR/Cas9. Genome engineering tools are based on the cutting of two strands at the target site of the target genome and subsequent repair of these sections through homologous recombination pathways or the connection of non homologous ends, thereby causing the desired genetic changes. In recent years, the CRISPR/Cas9 technique has been considered as a new and efficient method for genome editing and is based on bacterial immune system gene editing methods. It is noteworthy that this technique can be used to treat and control genetic diseases, improve food quality, make drugs, and in animals to improve growth and reproductive performance. The technology directly targets DNA and uses the Cas9 protein as a DNA-cutting molecule, which binds to the target sequence by a guide RNA that pairs with the DNA in question. This review discusses this new genome editing technique and its applications in mammals.