Abstract: Incremental improvements in our knowledge of human body composition are abetted by advances in research technology. Indeed, magnetic resonance imaging (MRI) represents a technological advance that has profoundly influenced body composition research. Routine applications of MRI include the measurement of whole-body and regional adipose tissue distribution, quantification of lean tissue and its principal constituent skeletal muscle, and the measurement of visceral adipose tissue. MRI is now the method of choice for calibration of field methods designed to measure body fat and skeletal muscle in vivo. Common to these applications is the measurement of tissue quantity. More recently proton (1H) and sodium (23Na) MRI protocols have been developed that measure the quality (lipid and sodium concentration) of skeletal muscle tissue. These unique applications of MRI represent a major advance in the study of altered muscle composition in vivo, with numerous applications in both applied and clinical medicine. In this review we provide a brief overview of routine applications of MRI in body composition research, followed by a focus on more recent applications of MRI that employ fast-imaging sequences for qualitative measurement of human skeletal muscle.