National standards for flammability have, in most countries, been established for many years. Thus, they are based on an understanding of fire physics which may be several decades old. In more recent times, however, significant strides have been made in developing test methods which are based on an improved understanding of building fires. The best-possible estimate of the flammability of building products or contents would be, by definition, a full-scale fire experiment. These are recently being standardized, and will serve an important reference function. Because of cost and practical difficulties, however, it is generally desirable to do the majority of product evaluations by bench scale tests. In this paper the rational bases for achieving validated bench-scale tests for flammability are examined, and a number of recent examples are cited where such a process has been followed. It is also shown that these test results have a correct relationship to the full scale phenomena.