Legislatures rely on rulemaking to add more detailed scientific, economic, or industry expertise to a policy — fleshing out the broader mandates of authorizing legislation.  Executive agencies are usually charged with executing, not making, the rules. Given the scope of modern regulation, however, legislatures frequently find areas where it is impractical for lawmakers to apply the level of detail or expertise required to establish complete standards. These they delegate to agencies for follow-up rule making.

The purposes of rule making include the following reasons:

• Adding scientific expertise
• Adding implementation detail
• Adding industry expertise
• Adding flexibility
• Finding compromise

Most modern rule making regimes have a common law tradition or a specific basic law that essentially regulates the regulators, subjecting the rule making process to standards of due process, transparency, and public participation.  In the U S, the governing law for federal rule making is the Administrative Procedure Act of 1946.  States have their own uniform administrative procedure statutes.