Although the role of nutrition as a therapy for the sick has been recognized for centuries, the science of nutrition is a relatively young discipline. The first modern attempt to document and standardize appropriate nutrition care was the diet manual. The evolution from “diet manual” to “practice manual” is less a change in purpose than an expansion of scope. This paper reviews the history of diet manuals in the United States, focusing on the evolution of nutrition therapy and the transformation of diet manuals into practice manuals for nutrition care providers. Included is a practice-oriented summary of 7 diet manuals published by Cleveland Metropolitan General Hospital in Cleveland from 1939 to 1984, when the hospital began using nationally distributed practice manuals. These manuals exemplify changes in the practice of medicine and in the role of nutrition providers on the healthcare team. A review of the evolution of clinical decision-making as documented in diet and practice manuals reflects increasing rigor in referencing scientific evidence. Nutrition therapies that seem quaint to us now reflect the traditional origins of many medical practices that persist today. Knowledge of this history should motivate us to critically evaluate the research base that supports all aspects of nutrition therapy, develop protocols to assess practices that remain unexamined, and embrace the discipline of evidence-based practice.