When it comes to legislative power, only Congress can exercise the legislative power. So, what's happening when agencies make rules and regulations? Aren't they exercising legislative power? They seem to be making general and prospective rules for the government of society. So, there may be a limit, a point, at which an agency's regulations actually becomes an unlawful exercise of legislative power. So, everything starts with Congress in administrative law. Congress has to enact a statute and then create the agency or department that is going to administer or enforce that statute. As part of the administrative agencies responsibility, agencies have the power to make rules and regulations to effectuate the statute enacted by Congress. But, those rules and regulations must always fit within the confines of the law and the legal requirements established by Congress. Additionally in the past, Congress tended to legislate in much more detail leaving much less discretion to executive departments. Today on the other hand, Congress increasingly delegates in very broad terms leaving much more discretion for agencies to issue rules and regulations and make national policies. Congress is not permitted to delegate its legislative power to agencies. This is called the non-delegation doctrine. Under the modern doctrine, the courts say that Congress must only provide an intelligible principle that the agency can follow. So long, as there is an intelligible principle agencies are allowed to make policies. They're allowed to make rules and regulations that govern individual conduct and that effect legal interests and private rights. But, there have been criticisms of this intelligible principle test, this standard, in the sense that the Court has only twice in all of its history struck down a congressional statute for not having an intelligible principle. In other words Congress enacts statues with increasingly broad terms but the Supreme court has found it almost impossible to police the line between impermissible and permissible delegations of legislative power.