This paper gives a review over methods to determine the degree of reaction for supplementary cementitious materials (SCMs) with focus on Portland cement-fly ash blends only and summarizes and highlights the most important findings which are detailed in a parallel paper published in Materials and Structures.Determination of the extent of the reaction of SCMs in mixtures is complicated for several reasons: (1) the physical presence of SCMs affects the rate and extent of the reaction of the ground clinker component-the so called "filler effect";(2) SCMs are usually amorphous with complex and varied mineralogy which make them difficult to quantify by many classical techniques such as X-ray diffraction;(3) the rate of reaction of SCMs in a cement blend may be quite different from its rate of reaction in systems containing simply alkali or lime.