Although errors in reasoning about conditional probability have been the focus of interest of psychologists for a long time, the development of conditional reasoning in school students has received little attention. This paper considers the responses of 69 students across grades 3 to 13 in an attempt to model the development of appropriate reasoning skills. Although acknowledging the misconceptions observed by earlier researchers, this study seeks a positivist approach to describe developing understanding by combining mathematical appropriateness with structural complexity. The results of the study are intended to inform other researchers, teachers, and curriculum developers. (Contains 13 tables and 6 figures.)