Loss of lean body develops from an imbalance in protein synthesis and catabolism and is associated with a variety of different disease and nondisease states, including severe malnutrition, cachexia, and physiologic age‐related loss (sarcopenia). Loss of lean body mass is prevalent among a significant proportion of the elderly population and has been associated with increased adverse clinical outcomes. Recognition of individuals at risk for low lean body mass may be difficult due to unequal distribution of losses across muscle and adipose compartments, and individuals who are both obese and sarcopenic demonstrate the highest risk for adverse events. Cross‐sectional imaging modalities provide an accessible and easily interpretable means of quantifying lean muscle content and are routine diagnostic tests for cancer patients. As a result, a growing body of literature has developed characterizing the importance of low lean body mass as a poor prognostic factor among cancer patients, regardless of age. Cancer patients, especially those with sarcopenic obesity, are at increased risk for treatment‐related toxicities from chemotherapy and increased overall mortality. Further investigations into the pathogenesis of muscle wasting among cancer patients are critical, as therapeutic oncologic interventions may inadvertently accelerate muscle catabolism. This review provides an overview of the definitions of low lean body mass, etiologic causes, clinical significance among cancer patients, and potential therapeutic interventions.