The concept of knowledge gap (the discrepancy in the amount of information acquired by different groups of people reached by mass communication) can be critically examined to consider its theoretical and practical implications in the context of using communication to bring about changes in society. Some causes of knowledge gap include differences in communication skills, the amount of prior knowledge, and the exposure and retention of information, as well as different communication modes. The two most important assumptions are that equitable distribution of knowledge is desirable and that the less privileged need to be helped more than the privileged. Other conclusions are that (1) the use of mass media ought to be combined with interpersonal communication, (2) the evaluation criteria of various campaigns ought to focus on the lower social class, (3) current media instruction programs need to be examined in relation to the knowledge gap concept, (4) knowledge gap studies may call for additional theoretical focus, and (5) it is not known whether the knowledge gap is a temporal or a relatively permanent phenomenon. (EL)