This study investigated how students develop their understanding of chemical representations with the aid of a visualizing tool, eChem, that allows them to build molecular models and simultaneously view multiple representations. Multiple sources of data were collected with the participation of high school students (n=71) over a six-week period. The results of the pre- and post-tests show that students' understandings of chemical representations improved substantially. The analysis of video recordings revealed that several features in eChem helped students construct models and translate representations. Findings suggest that models can serve as a vehicle for students to generate mental images, and that different types of 3-D models were not used interchangeably for these students. (Contains 46 references.) (Author/WRM)