Hmelo-Silver, C. E., & Barrows, H. S. (2006). Goals and strategies of a problem-based learning facilitator. Interdisciplinary Journal of Problem-based Learning, 1, 21-39. [Online]
Savery, J. R. (2006). Overview of PBL: Definitions and distinctions. Interdisciplinary Journal of Problem-based Learning, 1, 9-20. [Online]
Additional reading
Hmelo-Silver, C. E. (2004). Problem-Based Learning: What and how do students learn? Educational Psychology Review, 16, 235-266.
Hmelo-Silver, C. E., & Barrows, H. S. (2008). Facilitating collaborative knowledge building. Cognition and Instruction, 26, 48-94.