Background: Genome editing using CRISPR-Cas9 has become one of the basic methods of biological research over a short period of time. This recently discovered system of adaptive immunity of bacteria has been adapted to the needs of science and has become a valuable tool for DNA manipulation. Its simplicity and reliability have contributed to widespread use of the method. Genome editing refers to targeted modifications of genomic DNA with single base pair accuracy. CRISPR-Cas9 differs significantly from previous technologies in the simplicity of directing the enzyme to the target sequence. In the field of cancer research, CRISPR-Cas9 has enabled the development of a number of models for the study of carcinogenesis and drug testing. From a therapeutic point of view, CRISPR-Cas9 has been applied in the field of immunotherapy, especially in ex vivo genetic modifications of the T-cells of patients. Aim: Currently, several clinical trials are trying to verify the therapeutic potential of CRISPR-Cas9. Based on these studies, we have summarised the strategies used in the preparation of therapeutic tools useful in cancer therapy. Conclusion: CRISPR-Cas9 appears to be crucial in basic research, particularly in the study of the function of individual genes involved in carcinogenesis. However, it will still be necessary to optimise the efficacy, safety and specificity of CRISPR-Cas9 before it is used in clinical practice.