It is said that being a teacher is the most important profession in the world. To what extent is this true? The aim of this chapter is to show the enormous importance of teachers in education, student learning and performance, and their long-term impact on the labor market and the economy. We review factors such as teachers’ training, their cognitive and socioemotional capital, their performance and the channels through which their influence on students is produced, as well as the importance of their interactions with students, families, and colleagues for their own present and future performance. We analyze the role of teacher policies in improving their attraction, retention, and performance throughout their careers. This is developed mainly with evidence from the United States and other countries: unfortunately, economic research on teachers in Spain is practically non-existent. The chapter therefore reports on one of the factors, particularly the most important school factor, that most affect student and school performance. On the other hand, some econometric techniques recently applied to these questions will be presented. New strategies for exploiting and linking administrative data and instruments for measuring teacher quality and performance are also presented.