Many test-based accountability systems, including the No Child Left Behind Act of 2001 (NCLB), place great weight on the numbers of students who score at or above specified proficiency levels in various subjects. Accountability systems based on these metrics often provide incentives for teachers and principals to target children near current proficiency levels for extra attention, but these same systems provide weak incentives to devote extra attention to students who are clearly proficient already or who have little chance of becoming proficient in the near term. The authors show that, based on fifth grade test scores from the Chicago Public Schools, both the introduction of NCLB in 2002 and the introduction of similar district level reforms in 1996 generated noteworthy increases in reading and math scores among students in the middle of the achievement distribution. Nonetheless, the least academically advantaged students in Chicago did not score higher in math or reading following the introduction of accountability, and the authors find only mixed evidence of score gains among the most advantaged students. A large existing literature argues that accountability systems built around standardized tests greatly affect the amount of time that teachers devote to different topics. Results for fifth graders in Chicago, as well as related results for sixth graders after the 1996 reform, suggest that the choice of the proficiency standard in such accountability systems determines the amount of time that teachers devote to students of different ability levels. Appendices include: (1) Data Construction; and (2) Proof. (Contains 33 footnotes, 14 figures, and 1 table.) [This paper was presented at the "NCLB: Emerging Findings Research Conference" at the Urban Institute, Washington, D.C. on August 12, 2009.]