Three virtual worlds have been built to investigate the effect of immersive, multisensory computer-generated experiences on learning topics in science. Currently targeted at high school and beginning college students, these worlds address Newtonian mechanics, electrostatics, and molecular structure and dynamics. Data has been collected on usability and learning through questionnaires, pre- and post-tests, in situ prediction and experiment and post-session interviews. The results are not uniformly conclusive but suggest that students can improve their mastery of abstract concepts through the use of virtual environments that have been designed for learning. Moreover, usability studies have identified many significant problems that have been addressed in successive refinements of these worlds. Future work will include collaborative learning studies (both local and distant), use of intelligent agents, and comparison with two-dimensional microworlds.