In making bread flour is mixed with water and yeast. The yeast feeds on the small quantity of sugar contained in the flour, fermenting it and forming from it alcohol and carbon dioxide gas. The gluten being coherent and tough is blown into numberless small bubbles by the gas, which is thus retained inside the bread. On baking, the high temperature of the oven fixes these bubbles by drying and hardening their walls, and the bread is thus endowed with its characteristic porous structure. If a cereal meal devoid of gluten is mixed with water and yeast, fermentation will take place with formation of gas, but the gas will escape at once, and the product will be solid and not porous. Evidently from the baking point of view gluten is of the greatest importance. One of the most obvious methods that have been suggested for estimating the strength of wheat depends on the estimation of the percentage of gluten contained in the flour. The method has not turned out very successfully, for strength seems to depend rather on the quality than on the quantity of gluten in the wheat. Much attention has been given to the study of the causes of the varying quality of the gluten of different wheats. Gluten for instance has been shown to be a mixture of two substances, gliadin and glutenin, and the suggestion has been made that its varying properties are dependent on the varying proportions of these two substances present in different samples. This suggestion however failed to solve the problem. After seven years of investigation the author has worked out the following theory of the strength of wheat flours, which has finally enabled him to devise a method which promises to be both accurate and rapid, and to require so little flour that it can readily be used by the wheat breeder to determine the strength of the grain in a single ear. It has already been mentioned that a strong wheat is one that will make a large loaf of good shape and texture. The strength of a wheat may therefore be defined as the power of making a large loaf of good shape and texture. Evidently strength is a complex of at least two factors, size and shape, which are likely to be quite independent of each other. Not infrequently, for instance, wheats are met with which make large loaves of bad shape, or on the other hand, small loaves of good shape. Probably therefore the size of the loaf depends on one factor, the shape on another; and the failure of the many attempts to devise a method of estimating strength have been caused by the impossibility of measuring the product of two independent factors by one measurement.