Skeletal muscle and lean body mass may be vital to prognosis and functional recovery in chronic and acute illness, particularly in conditions in which muscle atrophy is prevalent. Ultrasound provides a precise and expedient method to measure muscle mass and changes in skeletal muscle at the bedside.Here, we describe the various methodological approaches along with the validation and reliability tests that have been performed in various populations. Current applications of ultrasound in chronic and acute illness as well as its limitations and strengths in quantifying the muscle mass and changes in muscle over time are discussed. To capitalize on the beneficial features of ultrasound for measuring muscle, we describe the work that is needed to optimize the usefulness of ultrasound in chronic disease and acute care.Given the precision, practicality, and ease of use, ultrasound is emerging as a highly useful tool in expediently measuring the muscle mass and changes in muscle tissue at the bedside. Ultrasound may be valuable in identifying patients who are at risk of malnutrition, in tracking muscle atrophy for the purpose of calculating nutrient delivery, and in assessing the success or failure of nutrition, pharmacological and rehabilitative interventions that aim to counter muscle atrophy.