Through a randomized controlled trial, this mixed-methods study evaluated changes in elementary science teachers' understandings, confidence, and classroom implementation of problem-based learning (PBL), inquiry, and nature of science (NOS) instruction following participation in a professional development (PD) as well as the components of the PD that teachers perceived facilitated these changes. Results indicated that following the PD, treatment teacher (n = 139) understandings of and confidence for teaching inquiry, NOS, and PBL were significantly greater than control teachers (n = 98) after controlling for preunderstandings and confidence. The effect sizes were large. Treatment teachers also incorporated significantly more PBL, inquiry, and NOS into their instruction. Modeling, microteaching with feedback and reflection, and in-classroom coaching facilitated teachers' confidence, understanding, and intention to implement the reform-based practices they learned. Implications for the understanding of the relationship between knowledge, confidence, and practice as well as elementary science teacher PD design are discussed.