This article discusses how the Food and Drug Administration has come to adopt informal guidance (agency advice that influences regulated entities but does not carry the force and effect of law) as its primary method of policymaking, as opposed to more formalized procedures like notice-and-comment rulemaking or case-specific adjudication. Using major developments in administrative law and modifications to FDA's regulatory regime as milestones, the article traces how and why FDA's use of informal guidance to fulfill its statutory mandate has changed over the past century. Along the way, the article identifies important doctrinal questions that persist today, namely (1) whether informal advisory opinions bind FDA and (2) the degree of judicial deference guidance documents should receive under the Supreme Court's decisions in Chevron and Mead. The article attempts to resolve these doctrinal ambiguities. It then undertakes a normative analysis of FDA's increasing reliance on informal guidance, and concludes that, on the whole, this development has benefited FDA's major stakeholders: regulated entities, the general public and the agency itself. The article closes with modest proposals for reform. The article features an appendix with several tables illustrating FDA's output of informal guidance documents by year, to facilitate further study.