Out-of-field teaching is a relatively unrecognized problem in schools. This digest reviews what research shows about the extent and causes of the problem. There is much disagreement about how much and which types of education and training teachers should receive and what credentialing requirements states should have. Analysts have used various measures of the issue, each resulting in different estimates of the extent of the problem. Considerable misunderstanding surrounds out-of-field teaching, especially regarding why so many teachers are teaching subjects for which they have little background. The source of out-of-field teaching generally lies in the lack of fit between teachers' fields of training and teaching assignments. Research suggests that teacher shortages are not a major contributor to the problem. A larger problem is how schools and teachers are managed. Principals have considerable discretion in staffing decisions and often find that giving teachers-out-of-field assignments is more convenient, less expensive, and less time consuming than the alternatives. Research shows that solving the problem of out-of-field teaching requires more than simply recruiting and training more candidates. It actually requires understanding how teachers are managed once on the job. (Contains 11 references.) (SM)