Oncology is the medical subspecialty dealing with the study and treatment of cancer. A physician who practices oncology is an oncologist. The term originates from the Greek onkos (ονκος), meaning bulk, mass, or tumor and the suffix -ology, meaning "study of."

Oncologists may be divided on the basis of the type of treatment provided.

• Surgical oncologists: who are surgeons who specialize in tumor removal.
• Medical oncologists: who deal with using medication or chemotherapy to treat cancer.
• Radiation oncologists: people who specialize in the treatment of cancer with radiation, a process called radiotherapy.
• N.B. In the UK, the majority of oncologists are known as Clinical Oncologists, and are fully qualified to practice both chemotherapy and radiotherapy. In most other countries these disciplines are more clearly segregated.

Oncologists may also be categorized on the basis of the patient type.

• Gynecologic oncologist: who specialize in the treatment of cancer in women. He/She can performs and give chemotherapy and assists in radiation therapy for these cancers in women.
• Pediatric oncologist: who specialize in the care of children with cancer.