This report provides an overview of concept and principle-learning studies and traces the theoretical basis for schema theory, which asserts that all knowledge is assimilated into a general cognitive framework. The procedures of an original study are described, which was designed to test the hypothesis that the inclusion of a "domain statement" in the explanation of a principle--that is, a statement of the range of applicability of the principle--would produce a higher level of understanding than would the presentation of a principle or example alone or the presentation of a principle in conjunction with an example. Although data, collected from 395 juniors and seniors from a middle-class and upper-middle-class public high school in suburban Chicago, did not confirm the major hypothesis, the findings of previous studies, which indicated that subjects perform better on test items that are similar and that a pretest facilitates posttest performance, were supported. Tables illustrating the text are included. (KS)