Under the No Child Left Behind Act (NCLB), every school is subject to the controversial mandates for annual test score gains contained in the federal law. The law represents a profound change in the relationship between the federal government and state and local education agencies regarding who controls education and has direct implications for what happens educationally in schools and classrooms. Although NCLB affects these and other important areas of the educational system and imposes great pressure on school leaders, it is silent on the role of principals in fostering school improvement. Yet many of NCLB's provisions have important implications for principals. The law is based on the assumption that external accountability and the imposition of sanctions will force schools to improve and motivate teachers to change their instructional practices, resulting in better school performance. By relying on the threat of sanctions and market mechanisms--choice and supplemental educational services--to force school improvement, the law tends to place the principals of low-achieving schools in the role of trying to produce very large gains every year for every subgroup of students. In this article, the authors highlight the contradictions and oversimplifications in the existing law. They also discuss the findings of a teacher survey which they conducted to understand teachers' views of the assumptions underlying NCLB and the implications of these findings for principals.