Skeletal muscle is well known for its locomotory and key structural functions. In addition, skeletal muscle provides essential roles for maintenance of metabolic health, acting as a storage point for glucose, lipids, and providing the largest store of amino acids in the body. As such, maintenance of muscle mass is key to the promotion of health and well-being across the lifespan and in many disease states. Skeletal muscle loss, in either clinical, or non-clinical situations, is strongly associated with increased physical disability, reduced quality of life, and mortality. Significant attempts are being undertaken to prevent or delay the loss of muscle mass in both healthy individuals and in clinical settings. Nevertheless, the underlying regulation of muscle mass remains poorly characterized, resulting in a lack of effective therapeutics to tackle muscle wasting. Stable isotope tracers enable the study of dynamic muscle mass regulation, with the potential to provide novel insights into the mechanisms governing skeletal muscle loss and paving the way for the development of more focused therapeutic approaches. This talk will focus on the principles of stable isotopes tracers and how they can be applied to investigate muscle mass regulation in health and disease.