Cancer is a rapidly proliferating cell group formed by the accumulation of numerous genomic disorders called mutations, which invades organs due to its invasive characteristics. These genomic variations, which also occur in normal healthy tissues, are detected and eliminated by the immune system. Cancer treatment, initially only surgical resection, has over time included radiation therapy, chemotherapy, targeted therapy, and immunotherapy, which excites the medical community and is the subject of intensive clinical research. The main mechanism of cancer immunotherapy is not to directly kill cancer cells like conventional chemotherapies but to reactivate the body’s immune system and reorganize the immune system to allow immune destruction of tumor cells. In advanced-stage and recurrent cancer patients, the ineffectiveness of cytotoxic therapy agents (chemotherapy) and the resistance developed by cancer cells against treatments through mutation have limited the success of these therapies. Modern cancer immunotherapy has started to take its place in the clinic from the second half of the twentieth century. Immunotherapy has achieved significant successes in the treatment of cancer since 2011. Alongside the beneficial outcomes of immunotherapies, long-term use can induce autoimmune reactions, while the development of resistance to immunotherapeutic agents represents a primary limitation. To overcome these limiting effects, there are intensive studies on biomarkers. There is not yet a fully ideal marker to predict which patient to give immunotherapy to, to predict treatment success, and the development of side effects. Despite the dizzying success observed in some patients with immunotherapy, for now, it falls short of promising great hopes for every type of cancer and every patient. However, with the right patient selection and the correct application strategy (alone, combined with other treatment agents, or sequential application), more encouraging results can be obtained every day.