Teachers vary widely in their ability to improve student achievement and the difference between effective and ineffective teachers have substantial effects, not only on standardized tests outcomes, but on later life outcomes (Chetty, Friedman, & Rockoff, 2011). As a result, educational practitioners, policymakers and researchers are searching for and experimenting with ways to improve the quality of teaching. Some of these efforts focus on teacher recruitment, under the belief that teachers with attributes observable at hiring may ultimately make better teachers. Some evidence supports this proposition, although most research uses blunt measures of teacher attributes and fails to identify attributes that meaningfully predict later effectiveness in the classroom. Others attempt to identify mentoring or professional development interventions that improve the knowledge and skills of the existing teaching labor force. Here again, some notable successes demonstrate the potential for improvement (Taylor & Tyler, 2011; Yoon, 2007), but most interventions appear to have little if any effect on teacher ability in the classroom. The general lack of understanding of the attributes, skills or behaviors of teachers that contribute to their effectiveness or of the factors that facilitate their improvement stands in contrast to what has become a stylized fact that, on average, teachers meaningfully improve during the first three to six years of their careers. What occurs during this period that makes teachers more become effective? Could teachers learn these lessons prior to entering teaching so that they are more effective in their initial teaching or could they learn the skills more quickly once they enter the classroom? The timing of their improvement could have meaningful implications for students especially given the high proportion of teachers who only teach for a few years. A variety of factors may affect teachers’ returns to experience, especially in the first five to ten years. These include their ability to learn, the extent to which teachers develop relevant skills prior to entering the classroom, the opportunities they have to learn, and the quality of the environment in which they find themselves first exercising their practice and learning “on the job.” The four papers in this session directly investigate these important aspects of teacher development. Atteberry et al and Kraft and Papay focus on better understanding the general returns to teaching experience as a way of improving the quality of teaching. Bastian et al. examine the returns to experience for science teachers. Goldhaber et al. examine the effectiveness of teacher preparation programs to better understand the differential abilities of teachers at entry to the profession. Together, the papers provide insight into the factors that naturally shape the quality of the teacher workforce, and lessons learned provide guidance for policy and future efforts to improve the overall distribution of teacher quality.