The prokaryotic type II CRISPR–Cas9 (clustered regularly interspaced short palindromic repeats–CRISPR-associated 9) system is rapidly revolutionizing the field of genetic engineering, allowing researchers to alter the genomes of a large range of organisms with relative ease. Experimental approaches based on this versatile technology have the potential to transform the field of cancer genetics. Here, we review current approaches for functional studies of cancer genes that are based on CRISPR–Cas, with emphasis on their applicability for the development of next-generation models of human cancer.