Several nutritional assessment tools have been used in oncology settings to monitor nutritional status and its associated prognostic significance. Body composition is fundamental for the assessment of nutritional status. Recently, the use of accurate and precise body composition tools has significantly added to the value of nutritional assessment in this clinical setting. Computerized tomography (CT) is an example of a technique which provides state-of-the-art assessment of body composition. With use of CT images, a great variability in body composition of cancer patients has been identified even in people with identical body weight or body mass index. Severe muscle depletion (sarcopenia) has emerged as a prevalent body composition phenotype which is predictive of poor functional status, shorter time to tumor progression, shorter survival, and higher incidence of dose-limiting toxicity. Variability in body composition of cancer patients may be a source of disparities in the metabolism of cytotoxic agents. Future clinical trials investigating dose reductions in patients with sarcopenia and dose-escalating studies based on pre-treatment body composition assessment have the potential to alter cancer treatment paradigms.