Does teacher professional development affect content and pedagogical knowledge: How much and for how long?

We examine the impact of teacher professional development on a key aspect of teacher quality--teacher knowledge. Specifically we use English Language Arts teacher content and pedagogy assessments to determine whether the California Professional Development Institutes significantly improve teacher content knowledge and whether teachers retain that knowledge six months after the institutes are completed. The results indicate that teachers vary significantly in pre-institute knowledge on the four assessed domains, that they demonstrate significant knowledge growth between the pre- and post-assessments, but that practical classroom experience hinders knowledge retention as measured six months later. Further, pre-exiting knowledge gaps are not systematically reduced thereby not alleviating the uneven distribution of teacher quality among California elementary schools.