Previous work has identified four areas of difficulty that students seem to have with the topic of similarity: (1) understanding the definition of similarity; (2) proportional reasoning; (3) dimensional growth relationships; and (4) correspondences in right triangle similarity. This paper reports the results of an investigation into high school students' understanding of similarity. A unit addressing three of these difficulties was constructed for use with the "Geometric Supposer." Students were observed as they learned similarity with this unit and were given pretests and posttests on fractions, ratio and proportion, and similarity. From the observations and tests, clarification of these three difficulties will be sought. The resulting greater understanding of student difficulties with similarity will be of use to practitioners and of interest to the mathematics education research community. The use of technology, specifically the "Geometric Supposer," provides two benefits. First, it supports a pedagogy which seeks to attack directly the students' difficulties in understanding similarity. Second, the lab setting allows researchers as well as teachers to examine directly student thought processes in the classroom. (Author/PK)