Public and scholarly debates on what it means to be a successful teacher are characterized by increasingly pronounced differences in how various political, legislative, and professional groups define successful teaching. In response to pressures posed by these polarized discourses, critical pedagogic research on education contrasts contested views on the nature of knowledge and critiques the standardization of knowledge in schools. There is a general consensus in the research that it is necessary for teachers to develop an understanding of a rapidly changing world, and an understanding of students coming from various social milieus who are often unwilling to accept a uniform set of knowledge and values it promotes. Thus, the body of knowledge and the values which are supposed to be passed on to new generations of students continue to be points of contention in education. It is necessary for teachers to be aware of why certain sets of knowledge, skills, and values should be promoted in increasingly diverse school settings. In this expanded view of the teachers' role and of teacher expertise, teacher agency, the nature of knowledge, sociopolitical relations, and power relations need to be considered. In this dissertation I examine the interplay of these factors by examining the extant research and then analyzing the perceptions of a group of teacher participants in a qualitative study. I have synthesized the findings to develop a new concept of teacher agency expertise in a critical pedagogic sense. I clarify the relationship between teacher expertise and teacher agency. The concept of teacher expertise requires that teacher agency be conceived of in terms of its collective expressions and its larger influence, as well as the aim of effective teaching for equity and social justice. [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.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]