Since the World Wide Web became in 1994 the first new mass medium since television, online learning design has evolved at Internet speed, taking in less than a decade what it took exhibit design over a century to develop in sophistication. Although virtual exhibits consisting of pictures and text are still common, educational Web designers increasingly employ techniques borrowed from interactive exhibit developers, video game producers, and museum educators to create compelling activities that fully exploit the strengths of the new medium. Constructivist learning theory often informs these new approaches. However, transplanting learning theory from the classroom or museum environment to the Web poses unique challenges. In this paper, we review several theories of learning and explore ways that we have tried to incorporate them into our development and design process for interactive Web sites. Constructivism underlies much educational practice in museums and is the basis for all of the learning theories we survey in this paper. Each of these, however, clarifies, expands upon, or revises the notion of constructivism in ways that can help Web designers better conceptualize and execute their projects. For example, Kolb's model of learning styles highlights the structure of the learning process. This model offers insight into how to make Web media go beyond the convergent/logical learning that comes easiest to computer-based learning, and to teach divergent, practical, and social learners. Similarly, Gardner's checklist of multiple entry points offers a valuable perspective on diversity in learning, prompting us to look for ways to engage various intelligences in one package. Most dramatically, Egan's notion of developmental "kinds of understanding" frees us from the strict constructivist demand to account for the concrete prior knowledge of our mostly anonymous online audiences. Instead of attempting that impossible feat, or ignoring the issue entirely, we can engage children's and adults' imaginative capacities with stories about profound abstractions, the limits of reality and experience, and our place in the world.