The decrease in muscle mass and strength represents one of the most relevant descriptor of physiological aging. Sarcopenia is the term coined to indicate the pathologic loss of skeletal muscle mass and strength/function during aging. The skeletal muscle decline has a multifactorial origin, involving lifestyle habits, disease triggers, and age-dependent biological changes. This phenomenon is part of the geriatric background and is today starting to disseminate in other specialties dealing with the complexity of frail older persons. In the oncology field, the interest in muscle wasting has mostly been focused on the clinical entity of cancer cachexia, a complex metabolic syndrome characterized by severe muscle loss, systemic inflammation, and malnutrition. The study of body composition in the oncological setting is crucial and may become one of the main characterizations of the oncogeriatric field, where clinical and research actions have to be designed taking into account the consequences of the aging process.