Abstract Regulatory agencies become involved in numerous "legal actions." Not only do these agencies initiate legal actions to enforce laws, they promulgate rules, issue statements of policy, and develop and define statutory standards. Not surprisingly, these programs are challenged in courts by members of regulated industry, trade associations, consumer groups, and individual citizens. Regulatory agencies also initiate and respond to a variety of issues by administrative, rather than judicial, action. These too are "legal actions" and frequently concern the adequacy of laboratory and scientific methodology. Where the agency initiates action to enforce the law, it carries the burden of proof. Where agency action or inaction is challenged, the challenging party carries the burden of proof and the courts have frequently given great deference to the views of the agency. However, the courts will frequently evaluate for themselves the scientific basis for the agency’s decision, regulation, or program. Examples of judicial reaction to scientific questions in recent years will be discussed.