Changes in medical terminology and disease taxonomy are an inevitable fact of life in medicine. As human innovation brings about new investigative tools and ever more powerful and discriminating analytical methods, inquisitive clinicians and medical scientists uncover novel medical facts at an unprecedented pace. Such new information not only expands the horizons of medical knowledge, it also challenges established concepts, brings into question old dogma, and shifts entrenched boundaries. Oftentimes the evolving understanding of disease processes cannot be fully reconciled with the existing medical knowledge without redefining and reclassifying old syndromes and diseases or, as is the case for entirely new medical conditions, defining and naming them. Sarcopenia illustrates the impact that constantly advancing medical knowledge has on disease definition and taxonomy. Sarcopenia also shows how a lack of clarity or consensus in a medical definition can affect drug innovation: in particular, how such a lack can influence academic research, drug development, and regulatory expectations. The term sarcopenia was coined in the 1990s. Its origin and significance are summarized by Irwin H. Rosenberg in a 1997 article referencing a presentation that he had made on April 17, 1966, in Washington, DC, at a symposium titled “Sarcopenia: Diagnosis and Mechanisms.”1 True to its Greek roots of sarx (“flesh”) and penia (“loss”), the term sarcopenia was coined to describe a decline in muscle mass occurring in the context of aging. Because structure and function are linked by indissoluble bonds, the relevance of sarcopenia has been from the very beginning connected with the notion of agerelated decline in muscle function, including but not limited to impaired ambulation and mobility. If anyone ever needs a visual confirmation of the concept of age-related muscle loss, one only needs to look at the article’s illustration of the difference in muscle size between a 24-year-old and a 64-year-old woman as shown on a magnetic resonance imaging cross-section of the thigh. And if a picture is worth a thousand words, the field of sarcopenia is today worth more than a thousand publications, commentaries, expert opinions, and several periodic expert meetings at national and international levels. Fifteen years later, many of the questions asked in Dr. Rosenberg’s thoughtful article are just as relevant, as are many