Body composition changes could indicate health conditions and potential health risks. Although several methods are currently available for quantitative assessment, each has limitations. Indirect methods are most frequently used because of their availability and simplicity. One of those methods, which found wide use in clinical trials and everyday clinical practice and was found to be of great value in oncology patients, is bioelectrical impedance (BIA). Assessed muscle volume and mass, measured by BIA, were associated with chemotherapy toxicity and overall prognosis, regardless of the primary tumor site. It has been shown that calculated phase angle could be a strong prognostic factor for a particular endpoint and an independent prognostic factor in patients with advanced malignancies. Body composition was also found to correlate with the quality of life (QoL) of patients with cancer, and several studies have shown a significant impact on various QoL subdomains. Measuring body composition and the information it provides could be used for the development of different clinical interventions that can help cancer patients live longer and better lives.