Abstract This paper describes the rule-assessment approach to cognitive development and summarizes work using it that is relevant to education. The basic question that motivated the rule-assessment approach is how people's existing knowledge influences their ability to learn. Addressing this issue requires a means of assessing what people already know, a means for studying how they learn, and a means for examining basic processes that may underlie developmental differences in knowledge and learning. Research using the rule-assessment approach is summarized in terms of eight conclusions about these issues. Each of the conclusions is illustrated with empirical examples.