Forty-five students divided into six experimental groups were given a programed presentation using teaching machines, which included different feedback procedures. The most efficient feedback procedure was that of administering a specific review until the criterion frame performance was correct. The least efficient procedure was that of repeating the previous presentation as many times as necessary until a correct performance was achieved. In a second experiment, one hundred and sixty school children in the second, third, and fourth grades demonstrated that although the hierarchy of concepts was logical it did not represent the skills in a sequence acquired by these children. Two related studies were also included although they were not a part of the project's proposal. The first investigated various combinations of right-wrong knowledge of results on conceptual learning. The results indicated that knowledge of results did not facilitate learning of the concept acquisition task. The second study examined the effect of the amount of negative knowledge of results upon naming of fractional amounts. Students receiving only the correct responses on the first trial showed significantly fewer errors in subsequent trials than the group which was given four alternative responses but no knowledge of responses on the first trial. (RR)