The goal of this review is to describe the mechanism of action and clinical applications of gene therapy. Gene therapy is a biological medical product which contains an active substance consisting of recombinant nucleic acid that is used in humans with the goal of regulating, repairing, replacing, adding or deleting the gene sequence. The transfer of exogenous genetic material into cells or transfection can take place ex vivo or in vivo. The ex vivo approach involves extracting cells or tissue from the patient, genetically modifying and cultivating the extracted cells or tissue outside the body and then returning them to the patient's body. The in vivo approach involves the application of gene therapeutics directly into the patient's body, distribution to tissues, transfection of target cells and genetic modification of these cells. Transfection methods can be physical, chemical and biological. Most gene therapy products used in clinical practice today use chemical (non-biological) and biological vectors in the form of lipid nanoparticles and viruses for transfection. Vectors are carriers of genetic material that protect and deliver nucleic acids to target cells. The aim of gene therapy can be to add genes, silence genes, edit genes or eliminate tumor cells. Gene addition is a method of gene therapy intended for the treatment of diseases in which the expression of the gene product is lacking or a non-functional product is created. The goal of gene silencing is to remove or reduce the expression of a gene because its product is overexpressed and/or toxic. Gene editing makes it possible to change the DNA sequence of a gene, thereby influencing protein expression, as well as the structure of the protein product at the genomic level. The method of eliminating tumor cells is based on the application of oncotropic viruses that induce cell death or enhance the host's immune response to the tumor.