Sarcopenia is defined as abnormally low muscle strength associated with reduced muscle mass, quality, and function. The prevalence of sarcopenia increases with advancing age, comorbidities, and disuse. However, sarcopenia can occur in the setting of various pathologic conditions in both older and younger individuals. Importantly, sarcopenia is associated with higher morbidity and mortality. With population aging, the estimated prevalence of sarcopenia is expected to rise to 200 million by 2050, and the clinical and economic impact of sarcopenia is expected to be substantial. Therefore, sarcopenia has become an important healthcare issue worldwide and remains an area of robust clinical and radiological research. Currently, the diagnosis of sarcopenia includes both clinical and imaging studies. Quantitative data on muscle mass can be obtained radiologically, including with whole-body dual x-ray absorptiometry, ultrasound, MRI, and CT. The objective of this book chapter is to provide a modality-based review of the quantitative imaging methods currently used to diagnose sarcopenia.