A culminating student teaching or internship experience is a central component of nearly every teacher education program and has been for most of teacher education's history. New teachers cite field experience and student teaching as the most beneficial, authentic, or practical aspect of teacher education. Teacher educators, however, have cause to view student teaching skeptically; student teachers often move away from the reform-minded practices espoused in teacher education. This multi-site ethnographic study investigated a full-year internship experience for six science interns at three diverse high schools as part of a teacher preparation program at a large state university. In taking an ecological perspective, this study documented the dynamic and evolving relationships between interns, cooperating teachers, teacher educators, and the school and classroom contexts. The goals of the study were to describe the changes in interns throughout the course of a year-long internship as a science teacher and to determine the relative influences of the various aspects of the ecology on interns. Data include fieldnotes from 311 hours of participant observation, 38 interviews with interns, cooperating teachers, and teacher educators, and 190 documents including course assignments, evaluations, and reflective journals. Interns' teaching practices were strongly influenced by their cooperating teachers. During the first two months, all six interns "used their mentor's script." When teaching, they attempted to re-enact lessons they witnessed their cooperating teachers enact earlier in the day. This included following the lesson structure, but also borrowing physical mannerisms, representations, anecdotes, and jokes. When interns could no longer follow their cooperating teacher due to an increased teaching load, they "followed their mentors' patterns"--implementing instruction that emphasized similar strategies--regardless of whether they were experiencing success in the classroom or not. To explore this disproportionate influence, this study documented the differences between the school-based placements and teacher education. Three contrasts were described. First, in schools, interns received support and assistance in real-time from cooperating teachers as they taught, while in teacher education, interns received support in planning for and reflecting on instruction. Second, in schools, interns and cooperating teachers' work had a task-orientation in which they solved concrete and contextualized problems together, while teacher educators were oriented toward ideas about teaching that might be generalized beyond the immediate context of the intern. Finally, in schools, interns acted like teachers. This meant dressing like a teacher, learning to use their bodies and voice in authoritative ways, and managing the physical space of the classroom. In teacher education classrooms, interns returned to talk and learn about teaching but reacquired the persona of students in their dress, movements, and social interactions. This study confirms the literature's consistent finding about the importance of cooperating teacher in the development of a student teacher's practices. In describing the worlds of the school and teacher education, it suggests reasons why the field experience acts as an influential "pedagogy of enactment" (Grossman, Hammerness, & McDonald, 2009) and discusses the implications for teacher education pedagogy. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]