For well over a decade, teachers have been recognized as the single most important school-level factor influencing student achievement (Darling-Hammond, 2000; McCaffrey, Lockwood, Koretz, & Hamilton, 2003; Rivkin, Hanushek, & Kain, 2000; Rowan, Correnti & Miller, 2002; Wright, Horn, & Sanders, 1997). Tremendous public resources have been invested across the country in new systems to both assess and address teacher effectiveness. Yet the lion's share of existing research seeking to understand the factors that make teachers effective focuses on quantifiable--yet more surface-level--inputs, such as years on the job, grade point averages, certifications, and degrees. This research leaves many important policy questions unanswered--particularly questions about the specific types of professional experiences and supports necessary to maximize teacher effectiveness. This report helps to fill these gaps through a first-of-its-kind exploratory survey of National and State Teachers of the Year. These teachers are chosen by their states as among the best, and one teacher each year is selected as among the best in the nation (see "Who Are the National and State Teachers of the Year?" on page 3). In this report, we offer insights on the professional experiences and supports that these teachers believe most contributed to their growth and eventual excellence as a teacher. Reflecting back on their careers, from preparation through mastery and even into retirement, the National and State Teachers of the Year surveyed in this study provide education leaders and policymakers with needed information about the formative experiences of great teachers. The findings both surprise us and confirm much of what we knew about how teachers evolve from good to great. The main body of the report takes readers through the four stages that this study used to define the teacher career continuum: Preservice, Novice, Career, and Teacher Leader stages. For each stage, it describes the professional experiences and supports that the National and State Teachers of the Year viewed as increasing teacher effectiveness as they progressed through the stages of their career. The report concludes with considerations for policymakers at the school, district, and state levels as well as for teacher preparation programs. Methodology for the survey is appended. [Proposed by the National Network of State Teachers of the Year (NNSTOY) and led by the Center on Great Teachers and Leaders (GTL Center) at American Institutes for Research, this study involved the following partner organizations, which collaborated extensively on this study, contributing to the development of the survey questions, data analysis, and drafting of the report: American Association of Colleges of Teacher Education, Council for the Accreditation of Educator Preparation, Council of Chief State School Officers, GTL Center, National Council on Teacher Quality, National Education Association, and NNSTOY.]