In this article, Heather C. Hill and Pam Grossman discuss the current focus on using teacher observation instruments as part of new teacher evaluation systems being considered and implemented by states and districts. They argue that if these teacher observation instruments are to achieve the goal of supporting teachers in improving instructional practice, they must be subject-specific, involve content experts in the process of observation, and provide information that is both accurate and useful for teachers. They discuss the instruments themselves, raters and system design, and timing of and feedback from the observations. They conclude by outlining the challenges that policy makers face in designing observation systems that will work to improve instructional practice at scale.