Luigi Cornaro (d. 1566) was a Venetian nobleman whose book De Vita Sobria (On the Temperate Life) was an instant success and has proved to be one of the most long-lasting and influential works of practical medical advice, counseling readers how to live long and healthily. Yet Cornaro was not a physician and his account raises a series of questions about the nature and location of medical expertise. Who can have that expertise? Can you, and should you, be your own physician, and, if so, on what grounds? I situate Cornaro's claims to expertise within a historically specific culture of medical dietetics in which personal experience counted for much. How did certain dietary practices "agree with" individuals? How did personal experience figure in constituting expertise? Was a healthy regime compatible with ordinary civic life and, if not, did it matter? What was the role of precise quantitative measure in prescribing the regime making for health and longevity? I address these questions with respect to Cornaro's historical setting and also in relation to pervasive commentary on his text over the centuries that followed.