Abstract: When an agency wishes to promulgate a rule, the default position under the Administrative Procedure Act (“APA”) requires public notice, an opportunity for comment, and the issuance of a “concise and general statement of basis and purpose.” The resulting documents are called “legislative rules” because they bind with the force of statutes. The APA, however, exempts “interpretative rules” and “general statements of policy” from notice-and-comment requirements. Those “nonlegislative rules” tend to grow out of relatively informal, often staff-centered policymaking processes to issue circulars, advice letters, guidance documents, staff manuals, and the like. Although such documents serve useful purposes of notice-giving and agency self-restraint, they also risk allowing agencies to make an end run around more formal notice-and-comment procedures. Accordingly, courts have distinguished the functions that legislative and nonlegislative rules may properly perform in the implementation of an administrative program. In particular, leading lower court cases suggest that an agency may not engage in legally binding policy making without invoking notice-and-comment procedures. On the one hand, an agency can engage in policymaking by issuing “general statements of policy,” but it cannot thereby bind itself or the public. On the other hand, an agency may issue a binding “interpretative rule,” but only if it can justify that rule as a mere interpretation of an antecedent statute or legislative rule as opposed to an act of independent policymaking. This article argues that these tests do not rest on judicially manageable standards. Because the line between interpretation and policymaking grows thin at the margins, the test for a valid interpretative rule almost surely reduces to a judgment that the rule does not reflect excessive policymaking discretion. That line-drawing problem presents judicial competence questions that are very similar to the ones that have provoked the Court to forbear from robust boundary policing in areas such as the nondelegation and the Chenery doctrines. After exploring the judicial administrability problems with the existing tests, the article considers alternative approaches to differentiating legislative from nonlegislative rules.