A mixed-methods research study was designed to examine teachers' knowledge and confidence for implementing a STEM-based problem-based learning (PBL) unit in their 6-12 grade science and math classrooms. Twenty-one teachers (7 in-service and 13 pre-service) participated in an intensive two-week summer workshop during which they engaged in, and then created, an immersive PBL unit related to sustainable energy. Data were collected through a pre-post content knowledge test and two pre-post surveys--one measuring knowledge and confidence for implementing PBL and one measuring science teaching efficacy. Daily reflections and focus group interviews provided additional data regarding teachers' changing knowledge and confidence related to both content and PBL methods. Results revealed significant gains in content knowledge related to concepts in energy, confidence for implementing PBL, and science teaching efficacy. Implications for the professional development of rural STEM teachers and the importance of engaging teachers in a professional development experience that integrates STEM content and PBL methods also are discussed.