The studies reported here are part of a research program whose purpose is to increase the effectiveness with which students acquire knowledge from written instructional materials. The studies had both a basic and an applied objective. The basic objective was to obtain evidence upon which to base a theory of the processes involved in language comprehension. The correlations between a large number of linguistic features and a measure of the difficulty students exhibited in comprehending the written language samples in which those features occurred were determined. The number of linguistic features which can be conceptualized numbers in the hundreds, most of which must be regarded as potentially representing a stimulus involved in the comprehension processes, because present theory of comprehension is too primitive to permit the authors to identify or to rule out more than a few of those features. The applied objective was to develop regression formulas for estimating if instructional materials are suitable for students of varying levels of language comprehension ability. These readability formulas provide a partial solution to the problem of fitting materials to students. That is, students may be provided with materials suited to their levels of comprehension ability not only by manipulating the materials to make them suitably understandable but also by selecting and using just those materials which are suited to the students' comprehension ability. (JL)