Quality teaching depends on a wide array of personal and contextual factors. An emerging and substantial body of literature has reframed the focus on teaching quality by examining how teachers' abilities to influence student achievement change over the course of their career and how these changes are influenced by the context in which they work. In this report, the authors review emerging research that can help answer the following two research questions: (1) How, if at all, do individual teachers grow in their ability to support student learning and development over the course of their career?; and (2) What conditions (e.g., access to professional learning opportunities, peer expertise) influence how teachers' abilities to support students change over time? To answer the two research questions, the authors conducted an exploratory review and analysis of empirical research related to teacher development. They limited studies included in the review to studies published in 2011 or later. Four studies for inclusion in this critical synthesis were selected, each exploring a unique variable related to teaching quality, including teachers' ability to influence students' achievement on academic assessments, teachers' effects on student academic behaviors (e.g., attendance), the relationship between teacher development and professional working conditions broadly, and the influence of quality collaboration on teacher development more specifically. This series of studies suggests that teachers continue to grow more effective in supporting important student outcomes--academic performance, attendance, reading behaviors--long after their first few years in the classroom and that teachers improve at greater rates when they work in supportive school conditions, including schools with opportunities for collaboration with colleagues and strong principal leadership. The authors urge state leaders to revisit and to rethink their approach to teacher policy under the Every Student Succeeds Act (ESSA). They argue that a large and growing body of research suggests that attention to "teaching quality"--the ability of teachers to support meaningful learning among students--is a promising approach for improving the quality and equity of educational opportunities in public schools. Policies focused on improving teaching quality attend to the conditions, resources, collaborative learning opportunities, and support from leadership that can accelerate the developmental trajectory of teachers and, in doing so, foster improvement among the teaching force broadly.