Examined in this article are the deference doctrines developed by courts in the United States, Canada, Britain, Australia and South Africa. Deference doctrines determine when and if courts are to defer to an agency's reasonable interpretation of the ambiguous terms of the statute that the agency administers. The study of deference doctrines in comparative perspective reveals much about the need for agency autonomy in the modern administrative state and the capacity of courts to maintain the delicate balance and remedy abuses of discretion. It also provides an opportunity to determine how well the leading theories of judicial decision making explain the variety of judicial responses to the common problem of deference to agency interpretation of statues.