This systematic literature review was conducted to explore the effectiveness of problem-based and project-based learning (PBL) implemented with students in early elementary to grade 8 (ages 3-14) in mathematics and science classrooms. Nine studies met the following inclusion criteria: (a) focus on PBL, (b) experimental study, (c) kindergarten to grade 8 level, and (d) focus on mathematics or science content. For these studies, we examined: the definitions of PBL used, the components of PBL explicitly identified as salient to student learning, and the effectiveness of PBL. This review found that although there is no consistent definition of PBL, PBL is an effective method for improving K-8 students' science academic achievement, including knowledge retention, conceptual development, and attitudes. Implications and limitations are discussed.