This article presents the results of a research project on the phenomenon of out-of-field teaching in American high schools - teachers teaching subjects for which they have little education or training. Over the past couple of years, the problem of out-of-field teaching has become a prominent topic in the realm of educational policy and reform, and the results of this research have been widely reported and commented on both by education policymakers and the national media. But unfortunately, out-of-field teaching is a problem that remains largely misunderstood. My research utilizes nationally representative data from the Schools and Staffing Survey, conducted by the National Center for Education Statistics. The purpose of this article is to summarize what my research has revealed about out-of-field teaching: how much of it goes on; to what extent it varies across different subjects, across different kinds of schools, and across different kinds of classrooms; and finally, the reasons for its prevalence in American schools. The data show that even using a minimal standard for qualified teachers - those holding a college minor in the fields in which they teach - the numbers of out-of-field teachers are striking. For example, a third of all secondary school teachers of mathematics have neither a major nor a minor in mathematics. My analyses have also shown that out-of-field teaching greatly varies across schools, teachers, and classrooms. The crucial question, however, and the source of great misunderstanding is why so many teachers are teaching subjects for which they have little background. I examine three widely believed explanations of out-of-field teaching - that out-of-field teaching is a result of either inadequate training on the part of teachers, inflexible teacher unions, or shortages of qualified teachers. My analysis shows that each of these views is seriously flawed. The article closes by offering an alternative explanation for out-of-field teaching - one focused on the organizational structure of schools and the occupational conditions and characteristics of teaching.