Chemotherapy is a systemic method of cancer treatment, in contrast with local therapies such as surgery and radiation therapy. The drugs used in chemotherapy are able to reach most parts of the body. Therefore, chemotherapy is likely to be recommended for cancer that has already spread to other areas of the body, for tumors that occur at more than one site, or for tumors that cannot be removed surgically. It is also used when a patient has recurrent disease after initial treatment with surgery or radiation therapy.

Chemotherapy is less mutilating than surgery and helps conserve organ or limb function since anti-cancer drugs are used to act on cancer cells without direct removal of a body part.

For some cancers, chemotherapy alone can destroy all the cancer cells and cure the cancer (primary treatment). As an adjuvant treatment, chemotherapy is given prior to, or after other methods, to increase the effectiveness of cancer treatment. Most often, adjuvant chemotherapy is given after other therapies have destroyed the clinically detectable cancer cells. The purpose of adjuvant chemotherapy is to reduce the risk of recurrence or to prolong survival. If cure is not possible, chemotherapy may be given to minimize the discomfort caused by cancer or slow the progression of the disease to prolong the patient's life (palliative treatment).

As a primary treatment, chemotherapy is used for some cancers such as Hodgkin disease, leukemia, Burkitt's lymphoma, localized diffuse large cell lymphoma, Wilms' tumor, small cell lung cancer, and testicular cancer.

Chemotherapy may be given prior to surgical resection or radiation therapy to shrink the tumor and make it easier to resect. This type of chemotherapy is called neo-adjuvant, induction, or preoperative chemotherapy.

As a palliative therapy, chemotherapy can be used to help make the cancer patient's life as comfortable as possible. In the case of Waldenstrom's macroglobulinemia, which is generally considered incurable, chemotherapy is administered to relieve symptoms and serious complications such as anemia.

Every cancer is unique, as is every cancer patient; therefore, the oncologist takes great care to tailor the chemotherapy plan to the particular case. The treatment protocol specifies what type of drug(s) should be given, what dosage should be given, how to administer the drug(s), how often the drug(s) should be given, and how long the treatment should last. During chemotherapy, the oncologist, who may change or modify the treatment plan to achieve better results, closely monitors the progress of the cancer patient and the tumor response.