The certification of an aircraft has progressed from demonstrations which were performed entirely upon the first-flight article to those in recent years which apply varying degrees of ground-based test facilities (simulators). The extent of these test facilities has been influenced by the complexity of the aircraft and the aircraft systems, the individual manufacturer's development philosophy and the funding allocated to simulation.This paper, based primarily on the development and certification of the Lockheed L-1011, concludes that increased simulation to show compliance is not only possible but in many instances is cost effective as well. This conclusion is based on a review of the requirements set down by FAR 25, the methods used to certify the L-1011 and the relative costs of compliance demonstration by flight testing and simulation.The present industry trend towards the derivative aircraft is making the argument for increased simulation for certification even stronger. The data base for the baseline aircraft has usually been well established by the time the derivative comes from the drawing board. Thus simulation, with supporting flight test, may well be the most cost effective means of certification.