Canine mammary tumors are treated mainly by operation, radiotherapy, chemotherapy and immunotherapy. Malignant tumor cells tend to metastasize to regional lymph nodes and / or distant tissues by lymphatics. In many cases, micrometastases have already been formed by the time of mastectomy, and in this case the operation alone will not interfere with the progression of the disease. It has been reported that operative administration will be easier with preoperative chemotherapy. Postoperative chemotherapy is a powerful adjunctive treatment that can improve disease-free survival and quality of life by controlling distant spread of the disease. There are many chemotherapeutics applied as adjuvant treatment in canine mammary tumors. It is possible to benefit from chemotherapy in dogs with mammary tumors by evaluating the general condition of the dog, correct histopathological diagnosis, selection of appropriate chemotherapeutic drug and appropriate dosing. However, different results were obtained regarding tumor regression and disease-free survival in different patients with the same histopathology tumor after the same chemotherapy protocols. One of the reasons why today's success in cancer treatment is not increasing very well despite the introduction of many new cancer drugs is the possibility of this different response. This demonstrates the necessity of individual treatment in cancer patients. Each cancer patient is different from one another at the molecular level. It is possible to determine the response of the patient's tissue to chemotherapy by using ATP-tumor chemosensitivity test in laboratory. Quantitative testing of the cytotoxic potential of chemotherapeutic agents applied to cells obtained from canine mammary tumor tissues will be able to develop individualized treatment methods. In this presentation, the chemotherapeutic agents used so far in canine mammary tumors, the use of chemosensitivity test in canine mammary tumors and the points to be considered in chemotherapy applications are mentioned.