Weight loss is common in patients with cancer. Many factors, such as physiological abnormalities, response to the tumour, and treatment, contribute to this weight loss. Cancer-related weight loss affects a patient's response to treatment, as well as survival and quality of life. Several nutritional screening and assessment tools have been developed for patients with cancer. This review describes the weight loss seen in patients with cancer as well as the methods of screening for nutritional deterioration and weight loss early in a cancer diagnosis. Nutritional approaches to the supportive care of patients with cancer are also discussed.