Biotherapy, often called biological therapy or immunotherapy, aims at supporting and helping in the treatment of human disease without chemical drugs and invasive therapies, by restoring the natural immune system. It is also used to reduce certain side effects that may be caused by some treatments against cancer, autoimmune diseases, or other diseases. Biotherapy employs substances called biological response modifiers (BRMs). The term BRM is often used synonymously with the terms immunomodulator and immunostimulant. BRMs are agents that modify the host's response to pathogens with resultant beneficial prophylactic or therapeutic effects. The use of BRMs had rapidly expanded since the introduction of the first diagnostic antibodies. They are now employed in oncology, autoimmune diseases, inflammatory diseases, and transplantation medicine. BRMs used in biological therapy include interferones, interleukins, colony-stimulating factors, monoclonal antibodies, differentiation agents, tyrosine kinase inhibitor, tumor necrosis factor, vaccines, and nonspecific immunomodulating agents. BRMs are widely accepted in the treatment of certain types of cancer and rheumatoid arthritis, while others are being tested in research studies. This article reviewed the clinical use and side effects of BRMs in cancer and other diseases.