Thesis advisor: Marilyn Cochran-Smith / Teachers have a powerful impact on student achievement, yet high attrition rates hinder schools in their ability to provide quality instruction. Attrition rates are highest for schools serving low-income, minority students and among small private schools, including Catholic schools. Attrition is especially prevalent among new teachers. Very few studies have focused on the problem of early leaving or the problem of retention in Catholic schools. This study seeks to understand better why public and Catholic school teachers leave teaching early. A mixed methods approach was used. This included 50 in-depth interviews with 15 public and 10 Catholic school teachers who left within the first 5 years. In addition, statistical analyses of public and Catholic school early leavers' responses in the Schools and Staffing and Teacher Follow-Up surveys were used to contextualize and compare the experiences of the 25 teachers interviewed to the larger population of early leavers. This dissertation argues that, to understand why teachers leave early, an approach that examines teachers' entire experiences throughout their short time in the profession is required. A framework informed by sociocultural and commitment theories and prior research on retention and the culture of schools was developed through systematic analysis of the interview and survey data. This analytical framework provides a complex approach for examining the phenomenon of early leaving, which included three aspects: entering commitment, teaching experience, and the decision to leave. Findings suggested that teachers' decisions to leave were influenced by multiple factors within their various contexts. These contexts and factors were constantly changing, making the decision to leave extremely complex. For Catholic school teachers, the decision was even more complicated, influenced not only by the same factors and aspects of early leaving as public school teachers, but also their changing identities as Catholics. Findings also called into question common assumptions about why teachers leave: teachers do not always leave because they are less committed to teaching, or are dissatisfied with teaching or with their salaries. / Thesis (PhD) — Boston College, 2009. / Submitted to: Boston College. Lynch School of Education. / Discipline: Teacher Education, Special Education, Curriculum and Instruction.