This report is the first in a series examining the magnitude of the teacher shortage and the working conditions and other factors that contribute to the shortage. It finds that the teacher shortage is real, large and growing. When indicators of teacher quality (certification, relevant training, experience, etc.) are taken into account, the shortage is even more acute than currently estimated, with high-poverty schools suffering the most from the shortage of credentialed teachers. This is significant because shortage of teachers harms students, teachers, and the public education system as a whole. Lack of sufficient, qualified teachers and staff instability threaten students' ability to learn and reduce teachers' effectiveness, and high teacher turnover consumes economic resources that could be better deployed elsewhere. The teacher shortage makes it more difficult to build a solid reputation for teaching and to professionalize it, which further contributes to perpetuating the shortage. In addition, the fact that the shortage is distributed so unevenly among students of different socioeconomic backgrounds challenges the U.S. education system's goal of providing a sound education equitably to all children. To address this problem, working conditions and other factors that are prompting teachers to quit and dissuading people from entering the profession need to be tackled. Additionally, extra supports and funding to high-poverty schools, where teacher shortages are even more of a problem, need to be provided to all schools.