Research documents that relatively few people can learn on their own from generic curricular materials, absent instructional customization and social support. This outcome is independent of medium: correspondence, audio, video, online, and elaborate multimedia courses all show a similar pattern. To succeed, the vast majority of learners need encouragement from other people to be tenacious when they are struggling, and require tailoring of the knowledge and skills presented to their interests, strengths, and weaknesses. When experiences are "massive," skilled human teaching to personalize each person's learning is not possible. Designers can embed self-activated forms of generic customization (e.g., hint systems, choosing the sensory channel on which information is presented, gaming and badging for engagement, sidebars that speak to particular interests). These can provide minor help for learners, but rely on participants' motivation and selectivity in using them and typically do not achieve major improvements in persistence or achievement. A powerful method of personalization that is potentially empowered in massive experiences is interaction and collaboration among learners. This session will brainstorm ways to design a massive learning experience in which peer interaction provides a high degree of personalization, as well as a powerful lens for studying various types of collaborative knowledge-building.