Malnutrition is a common comorbidity in patients with cancer, with determinants linked to the patients, the tumor, and the treatment. Cancer malnutrition, also called cachexia, represents the chronic form of disease-related malnutrition with chronic inflammation, and strongly impairs the prognosis. Reduced muscle mass, otherwise called sarcopenia in these patients, is a feature of malnutrition and thus of cancer cachexia that may appear before weight loss, and has a strong negative effect on prognosis, as well. The reported global prevalence of loss of muscle mass is 39%, whereas the prevalence of malnutrition in important cohorts varies from 25% to 70%. This is mostly due to the very different screening and diagnostic tools used throughout the world. The recent Global Leadership Initiative on Malnutrition has become a consensus alternative to standardize how malnutrition may be diagnosed based on etiologic and phenotypic criteria available everywhere, from rich to poor countries. This will lead to an easier diagnosis of cancer malnutrition that itself will help us speak the same language worldwide.