Due to the student-centered nature of problem-based learning (PBL) and project-based science (PBS), it is easy for teachers "not" to provide students with adequate feedback or enough support to promote critical thinking. However, research has shown that PBL and PBS are most effective when appropriate learning goals are defined, embedded supports and feedback are part of instruction, and there are multiple opportunities for self-assessment and revision (Barron et al. 1998). Instructional supports for student learning come in many forms. In this article, the authors describe how formative assessment can be used to support PBL and PBS to maximize student achievement. (Contains 7 figures.)