Experimental studies in the last quadrennium that are applicable to metamorphic petrology are many and varied. They range from the rigorous formulation of P‐T diagrams to thermochemical studies and solubility measurements. One encouraging feature of the large number of studies is an attempt to correlate the various studies with those of other workers. Such efforts require sound theory, and several important steps have been made in the past 4 years.Chayes[1968] has shown how discriminant functions can aid in the determination of field boundaries from scattered experimental points.Thompson[1967] andGreen[1970] have contributed toward the analysis of nonideal solutions.Greenwood[1967a] andEugster and Skippen[1967] have contributed to methods of handling gas equilibria. Estimates of pressures and temperatures of metamorphism have been made through the measurement of piezobirefringent effects of quartz and sillimanite inclusions in almandine byRosenfeld[1969] andAdams et al. [1970].