The age of statutes has given way to an era of regulations, but our jurisprudence has fallen behind. Despite the centrality of regulations to law, courts have no intelligible approach to regulatory interpretation. The neglect of regulatory interpretation is not only a shortcoming in interpretive theory but also a practical problem for administrative law. Canonical doctrines of administrative law – Chevron, Seminole Rock/Auer, and Accardi – involve interpreting regulations, and yet courts lack a consistent approach. This Article develops a method for interpreting regulations and, more generally, situates regulatory interpretation within debates over legal interpretation. It argues that a purposive approach, not a textualist one, best suits the distinctive legal character of regulations. Administrative law requires agencies to produce detailed explanations of the grounds for their regulations, called statements of basis and purpose. Courts routinely use these statements to assess the validity of regulations. This Article argues that these statements should guide judicial interpretation of regulations as well. By relying on these statements as privileged sources for interpretation, courts not only grant deference to agencies but also treat these statements as creating commitments with respect to a regulation’s meaning. This approach justifies a framework for interpreting regulations under Chevron, Seminole Rock/Auer, and Accardi that is consistent with the deferential grounding of these doctrines, and provides more notice to those regulated than does relying on the regulation’s text alone. This Article also shows how regulatory purposivism constitutes a new foothold for Henry Hart and Albert Sacks’s classic legal process account of purposivism. Hart and Sacks’s theory is vulnerable to the criticism that discerning statutory purpose is elusive because statutes do not often include enacted statements of purpose. Regulatory purposivism, however avoids this concern because statements of basis and purpose offer a consistent and reliable source for discerning a regulation’s purpose. From this perspective, the best days for Hart and Sacks’s legal process theory may be ahead.