The predicted thermal-hydraulic response of reactor and containment systems to postulated severe accidents is useful in determining mitigative features or emergency procedures. Subscale experiments provide data for ongoing development and validation of computer programs for tracking severe accidents. This paper describes a procedure for obtaining scale model laws useful in the design of subscale tests which are representative of full scale behavior. Example phenomena considered include molten debris discharge from a vessel, spreading flow on a horizontal floor, simultaneous convective cooling by an overlying water layer and conduction to the floor, debris immobilization by freezing, and thermal response of the steel containment shell following debris arrival.