A working paper from MDRC builds on and updates a literature review of project-based learning (PBL) published in 2000. Focused primarily on articles and studies that have emerged in the last 17 years, the working paper discusses the principles of PBL, how PBL has been used in K-12 settings, the challenges teachers face in implementing it, how school and local factors influence its implementation, and what is known about its effectiveness in improving learning outcomes.The report suggests that the evidence for PBL’s effectiveness in improving student outcomes is “promising, but not proven.” The biggest challenge to evaluating the effectiveness of PBL, the researchers suggest, is a lack of consensus about the design of PBL and how it fits in with other teaching methods. Some studies have found positive effects associated with the use of PBL. However, without a clear vision of what a PBL approach should look like, it is difficult for teachers and schools to assess the quality of their own implementation and know how to improve their approach. They also suggest that PBL implementation is particularly challenging because it changes student-teacher interactions and requires a shift from teacher-directed to student-directed inquiry, and requires non-traditional methods of assessment.The paper concludes with recommendations for advancing the PBL research literature in ways that will improve PBL knowledge and practice.