This paper presents research on participant learning processes in challenge course workshops using the framework known as Cultural-Historical Activity Theory (CHAT). CHAT views learning as a shared, social process rather than as an individual event. Participants' experiencing and learning was mediated by the physical and social conditions of the experience and by the contributions of other participants. The concept of mediation suggests that the meaning participants make of experience is not an individual event, but instead is enacted as a creative, collaborative process using cultural and institutional tools. The recognition that people's physical, social and reflective learning processes are mediated, challenges longstanding assumptions about the radical autonomy of learners, about "direct experience," and about the centrality of independent, cognitive reflection in experiential learning. Empirical data showing processes of mediation are presented, and the implications for research and theory are discussed. (Contains 3 figures and 1 note.)