The use of computer-based tools in education is booming, and governments are pouring large amounts of funding into providing these tools for teachers, while the evidence for their educational effectiveness is still being gathered. This study compared the effectiveness of scientific visualizations - computer-based animations and simulations - with teacher's 'normal' teaching without visualizations, in terms of students' development of key chemistry and physics concepts. In a cross-over research design, the same classes - students and teacher - completed one concept using scientific visualizations and one concept without, while other classes reversed the sequence. Over all, there were no significant differences in conceptual development between the students taught with and without visualizations. While this is in a sense a null finding, there is a significant literature that indicates that students enjoy learning with visualizations, so given that they enhance enjoyment and 'do no harm' in terms of learning, there is a case to be made for their use in teaching. That case should probably not, though, at least based on these results, be made on the basis of extravagant claims about enhancing student learning.