<p>Frailty in older patients with cancer has been associated with functional impairments, decline in quality of life, and increased risk of mortality. There is growing interest in prehabilitation interventions designed to optimize function prior to cancer treatment to mitigate functional decline and to optimize post-treatment outcomes. This review aims to describe the heterogeneity in muscle wasting definitions, modalities used for body composition analysis, and functional outcomes investigated in exercise prehabilitation trials in cancer patients. Defining muscle loss among patients with cancer is needed to better diagnose, treat, and prevent functional decline. Utilization of a consensus on definitions for muscle wasting syndromes is critical to evaluate the efficacy of prehabilitation and exercise interventions in cancer care.</p>