In several diseases, low muscle mass has been revealed as an unfavorable prognostic factor for outcome. Whether this holds true in patients with solid malignancies as well has increasingly been explored recently. However, this research field is severely hampered by a lack of consensus on how to determine muscle mass in cancer patients and on the definition of low muscle mass. Consequently, the prevalence of low muscle mass varies widely across several studies. Nevertheless, most studies show that, in patients with solid malignancies, low muscle mass is associated with a poor outcome. In the future, more research is needed to get better insight into the best method to determine muscle mass, the exact prognostic value of low muscle mass in diverse tumor types and stages, pathophysiology of low muscle mass in patients with cancer, and ways to intervene and improve muscle mass in patients. This review addresses the current literature on the importance of muscle mass in cancer patients and the methods of muscle measurement. Implications for Practice: An increasing number of studies underline the clinical value of low muscle mass as a prognostic factor for adverse outcomes in cancer patients. However, studies show large heterogeneity because of the lack of a standardized approach to measure muscle mass and the lack of reference populations. As a result, the interpretation of data and further progress are severely hampered, hindering the implementation of muscle measurement in oncological care. This review summarizes the methods of diagnosing low muscle mass in cancer patients, the difference between underlying syndromes such as sarcopenia and cachexia, and the association with clinical outcomes described so far. Many studies underline the clinical value of low muscle mass as a prognostic factor for adverse outcomes in cancer patients, but there is no standardized approach for measuring muscle mass. This review summarizes the methods of diagnosing low muscle mass in cancer patients, the difference between underlying syndromes (sarcopenia and cachexia), and the association with clinical outcomes.