This study examined the collective teacher efficacy in selected elementary schools, specifically exploring the extent to which those schools demonstrate collective efficacy, the nature of collective efficacy in those schools, and the underlying belief systems and tacit assumptions that may inform that efficacy. The first phase of a mixed methods design was used to determine the level of collective efficacy present in eight diverse elementary schools. Through a series of interviews in the second and third phases of the study, I investigated the nature of collective efficacy in those schools and the possibility that underlying belief systems such as deficit thinking may inform beliefs at the school level. My respondents came from eight elementary schools in three northern Illinois school districts serving increasingly diverse student populations. Results from phase 1 of the study provided a range of collective teacher efficacy (CTE) scores for the participating sites and revealed no significant variance in the CTE scores of the eight schools. In phase 2 of the study, interviews were conducted with six building administrators in four selected schools to explore the nature of collective efficacy in their schools and to examine the underlying belief systems from the vantage point of school leaders. In phase 3 of the study, teachers from two schools facing different school-based challenges but with similar collective efficacy scores and similar demographic data were interviewed to provide a more comprehensive understanding of collective efficacy and underlying belief systems in those two schools. As supported in previous research, collective teacher efficacy did correlate with student achievement despite varying levels of socioeconomic status. The eight school sites participating in phase 1 of the study demonstrated levels of collective teacher efficacy that correlated with socioeconomic status. Despite slight differences in the CTE scores, with all schools scoring in the average range, no significant variance between the scores were found. Given the complexity of increasingly diverse school populations and the limitations of academic achievement data as an independent variable, quantitative analysis alone was not adequate to examine the construct of collective efficacy in these schools, given their common selection criteria. The qualitative findings of this study revealed that belief systems grounded in deficit thinking coexisted with average levels of collective teacher efficacy. That collective teacher efficacy can coexist with deficit thinking is problematic for school improvement efforts based on collective efficacy theory. The utility of collective efficacy as a framework for school improvement is questionable for school leaders working to realize a socially just education and to understand the issues confronting schools serving diverse populations. A deeper interrogation and a measurement of deficit thinking needs to be conducted to better articulate the relationship between collective efficacy and deficit thinking in elementary schools serving increasing diverse student populations. [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.]