In this study, we discussed the participating teachers' implementation and understanding of science, technology, engineering and mathematics (STEM) project based learning (PBL). A research team at a STEM center systematically offered professional development activities to 92 teachers in an urban school district in the southern U.S. To investigate the teachers' implementation and understanding of the STEM PBL pedagogy, we conducted a collective case study with five teachers. Study data included interviews, in-class observations, and lesson plans designed and implemented by the teachers. The results of this study indicated that the PD sessions were effective in communicating several important concepts about STEM PBL. Nevertheless, our observations revealed that five teachers' classroom enactments did not necessarily convey their understanding of STEM PBL.