Clinical practice guidelines are now ubiquitous. This article describes the emergence of such guidelines in a way that differs from the two dominant explanations, one focusing on administrative cost-cutting and the other on the need to protect collective professional autonomy. Instead, this article argues that the spread of guidelines represents a new regulation of medical care resulting from a confluence of circumstances that mobilized many different groups. Although the regulation of quality has traditionally been based on the standardization of professional credentials, since the 1960s it has intensified and been supplemented by efforts to standardize the use of medical procedures. This shift is related to the spread of standardization within medicine and especially in research, public health, and large bureaucratic health care organizations.