Contemporary educational theory holds that one of the pivotal causes of inadequate student achievement, especially in disadvantaged schools, is the inability of schools to adequately staff classrooms with qualified teachers. Deficits in the quantity of teachers produced and in the quality of preparation prospective teachers receive have long been singled out as primary explanations for underqualified teaching. In this study, I hypothesize that the manner in which schools are organized and in which teachers are utilized can account for as much of the problem of underqualified teaching as do inadequacies in teacher training or the supply of teachers. This analysis specifically focuses on a little recognized source of underqualified teaching--the problem of out-of-field teaching--teachers being assigned by school administrators to teach subjects that do not match their training or education. I use data from the Schools and Staffing Survey--a large, comprehensive, nationally representative survey of teachers conducted by the National Center for Education Statistics. The results show that while most teachers, even in disadvantaged schools, hold basic qualifications, a significant proportion of these qualified teachers, especially in disadvantaged schools, are assigned to teach classes out of their fields. Data also show that out-of-field teaching is not primarily due to school hiring difficulties resulting from teacher shortages. In contrast, the analysis shows that a number of aspects of the administration and organization of schools are significantly related to out-of-field teaching. For example, school district regulations concerning minimal education requirements for new hires, the quality of principal leadership, the strategies schools use to cope with teacher recruitment and hiring, and average school class sizes all have an independent association with the extent of out-of-field teaching in schools, after controlling for other factors. (Contains 3 figures, 6 tables and 13 endnotes.)