Preoperative assessment of the cancer patient can be broken down into: (1) What is it? (2) Where is it? and (3) What is the status of the patient? Evaluation of local disease begins with obtaining and interpreting cytologic or histologic (biopsy) samples. Tumor staging involves the assessment of tumor invasion into local tissues, draining lymph nodes, and distant sites, especially the pulmonary parenchyma. Staging is performed with the aid of imaging techniques and special procedures such as lymph node and bone marrow aspiration. Because many veterinary cancer patients are geriatric, examination of physiologic patient status through laboratory and other diagnostic avenues is vital to the appropriate selection and success of therapy. After these questions are answered, appropriate treatment options and an ultimate treatment plan can be formulated. This article describes logical and practical approaches for diagnosis, staging, patient assessment, and treatment planning of the animal presented with tumor disease. With appropriate preoperative assessment, we can offer our patients and clients the best care possible.