The quality of teachers is now a central focus of policy and research. In spite of this focus, two key gaps exist, and this dissertation helps to fill them: First, mediation models involving the study of the effects of teaching are rare; instead, most current investigations of teachers focus primarily on the effects of teachers, with less attention to the contexts that may impact the quality of teaching practice. This dissertation considers how teaching practice mediates the effects of key contextual factors in the school environment, specifically the social capital available to teachers. By considering the impacts of teacher social capital as well as those of teaching practice, this dissertation broadens the scope of the investigation into how teachers might impact student learning. Second, research on teachers has been conducted using widely different instruments and models, and there is a lack of knowledge about the consequences of particular choices of measures and modeling assumptions. This dissertation compares the relationships among teacher social capital, teaching practice, and student achievement across single- and multilevel models and models employing latent and manifest factors/indicators. In addition, this dissertation offers conceptual and empirical analyses of the survey items included in the instruments and a comparison of inferences between expert observation ratings and student survey responses to better understand what aspects of teaching are adequately reflected in the measures employed and how this may influence the estimated relationships.