The concept of health literacy evolved from a history of defining, redefining, and quantifying the functional literacy needs of the adult population. Along with these changes has come the recognition that sophisticated literacy skills are increasingly needed to function in society and that low literacy may have an effect on health and health care. We present a brief history of literacy in the United States, followed by a discussion of the origins and conceptualization of health literacy. Increased attention to this important issue suggests the need to review existing definitions of the term "health literacy," because despite the growing interest in this field, one question that persists is, "What is health literacy?"