Project-based science (PBS) aligns with national standards that assert children should learn science by actively engaging in the practices of science. Understanding and implementing PBS requires a shift in teaching practices away from one that covers primarily content to one that prompts children to conduct investigations. A common challenge to PBS implementation is a misunderstanding of the elements of PBS. Identification of these misunderstandings as well as implementation challenges could inform professional development. This case study examined 24 teachers' understanding and implementation of PBS during participation in a consecutive three-year, comprehensive professional development program. Results provide insight as to the process they followed in the transition to implementing PBS. Measures included classroom observations, reflective interviews, and attitudinal surveys. Results showed that teachers developed the knowledge, confidence, and understanding to implement PBS but in most cases it took at least two to three years for positive results to become evident. Teachers struggled to develop adequate driving questions that provided project-focused lessons. Other obstacles included teacher resistance to student-directed instruction, confusing inquiry-based instruction with hands-on activities, and inability to motivate students to work in collaborative teams. While challenging, over time the teachers developed the knowledge, desire, and skills to implement PBS.