When students return to school this year, many will enter one of the more than 100,000 classrooms across the country staffed by an instructor who is not fully qualified to teach. This is because many districts, facing ongoing teacher shortages, are hiring underqualified candidates to fill vacancies. While shortages tend to draw attention to recruitment issues, this report finds that 90% of open teaching positions are created by teachers who leave the profession. Some are retiring, but about 2/3 of teachers leave for other reasons, most due to dissatisfactions with teaching. Teacher attrition in the United States is about twice as high as in high-achieving jurisdictions like Finland, Singapore, and Ontario, Canada.