Web Weather for Kids is a web site developed at the University Corporation for Atmospheric Research to interest middle school students in dramatic weather phenomena and weather prediction. The original web site was awarded the 2000 Unisys Prize for Online Science Education as a "model for collaboration and a powerful example of how the web can meaningfully affect classroom practice." In 2000, Web Weather for Kids received a grant from the NSF Geoscience Education program enabling its expansion to a set of interactive learning modules covering basic science concepts, thunderstorms, tornadoes, clouds, blizzards, and hurricanes. Students learn through several modes, such as quizzes, animations, interactive simulations, stories, games, and hands-on activities. The development team consists of scientists, educators, a web design artist, and an evaluator experienced with self-contained learning tools. Formative evaluation has been integral to development. The development team regularly engaged in dialog about the learning goals of each module, best practices for achieving those goals, and creative new ideas for engaging and motivating middle school children. As a "science novice," the evaluator raised issues from the viewpoint of someone learning the concepts for the first time, often leading to changes in approach. Between meetings, evaluation data were generated through college student use of the web site; based on these data, the team would decide what changes to make. Five representative middle school students were enlisted to use a beta version of the site in UCAR's usability lab. Their feedback and excellent ideas led to other changes. During summer 2002, five teachers have been engaged to provide in-depth feedback in terms of content, design, grade level appropriateness, and utility for classroom teachers. Over the 2002-2003 academic year, teachers will use the site in real classrooms, where student outcome data will be generated. Pre- and post-tests, combined with observations and interviews, will provide understanding of what students learn. This presentation will describe the lessons learned from formative and summative evaluation, from the regular evaluation dialogs to usability testing with students to the site's efficacy as a learning tool based on teachers' evaluations and student outcomes.