In this study we examined changes in K-12 science teachers’ self-efficacy and teaching practices, while participating in a year-long problem-based learning (PBL) professional development (PD) program. The teachers who completed self-report surveys at the beginning and the end of their PBL PD program reported that they developed a greater sense of confidence in their ability to teach effectively and a stronger belief that their teaching of science would result in student learning. Results also indicate significant improvements in teachers’ use of (1) standards-based teaching practices, (2) informal assessments, (3) projects, (4) communication, (5) calculators/computers, (6) journals, and (7) the Internet. The study also examined various teacher characteristics underlying changes in teachers’ self-efficacy and teaching practices