The mental capacity of adult persons is indicated rather accurately by the number and variety of things they have learned from the school of life, omitting from the count so far as possible those things directly taught by the formal schools. The intellectual capacity of an infant is likewise indicated roughly by the changes which his brief experience in life has brought about in his ability to control himself and his immediate surroundings. At birth the child is practically without control of his own body, except for certain sucking reflexes, certain crying reflexes, and the reflex which causes it to grasp with its fingers or toes the finger or pencil which is brought into contact with them. These reflexes can hardly be said to be a part of the child’s control of his own body, for the child could not avoid obeying them if he so desired. The new-born infant is practically helpless even so far as his own body is concerned. Any degree of control he may later develop over his body or over other things outside is attained because he has a system of nerve and brain cells capable of forming connections and being modified by these connections and their results. The degree to which the child’s nervous system is organized and supplied with potential connections by inheritance is reflected very early in life by the quickness with which it learns to control itself and its immediate environment. The most ready method of testing this inherited capacity is therefore to determine at any particular age just how complex and adequate a system of control has been developed. The tests for measuring infants and young children consist, then, in those performances under specified standard conditions, which are typical for children at various ages. The Mentimeter for young children is based directly upon the findings of Prof. Lewis M. Terman and his students in their investigations of children from three to twenty years of age. This investigation was so carefully devised and executed that to improve on the product, the Stanford Revision of the Binet tests, would be almost impossible. The tests for children three and four years of age presented in the following pages are therefore not claimed to be new or original with the present authors, who hereby express to Doctor Terman their admiration for his excellent contributions in the field of psychological measurements. The tests here suggested for children under three years of age are modifications of those suggested by Doctor Kuhlmann. All investigators in this field are, of course, indebted to the pioneers, the French psychologists, Binet and Simon.