Monoclonal antibodies are a promising and rapidly growing class of biological drugs that are among the best-selling on the market. They are a part of immunotherapy, which, along with chemotherapy, surgery and radiation, is one of the basic methods of cancer treatment. Immunotherapy began to develop when the understanding of the relationship between cancer and the immune system progressed, and soon after that, the development of monoclonal antibodies followed. Antibodies are glycoproteins that are produced in the body from B-cells and their physiological role is defending and protecting the body from pathogens. They act by binding to an antigen or Fc receptor and thus mediate various executive functions - stimulating immune cells to destroy pathogens, opsonizing pathogens and activating the complement. Monoclonal antibodies were first produced in the 70s by the hybridoma technique, which made it possible to obtain a large number of identical antibodies specific for a certain antigen. For therapeutic purposes, they are used as a kind of substitute for physiological antibodies in order to improve, modify or renew the immune system's attack on malignant cells. Further progress in technologies for their production and humanization, such as the use of transgenic mice or phage display, facilitated the creation of highly specific drugs with low immunogenicity. Monoclonal antibodies bind the target tumor antigens with high affinity and specificity and induce anti-tumor immune responses. They come in several forms, and can be divided into three categories: unconjugated, conjugated and bispecific. Their diverse mechanisms of action provide effective medical treatment and fight against cancer and many diseases. However, the use of monoclonal antibodies carries the risk of adverse reactions such as cytokine storm, serum sickness and toxicity to some organs. In current preclinical and clinical research, efforts are being made to reduce these side effects to a minimum.