Teachers want to succeed, but they typically lack the conditions for success. Teachers generally work in isolation. They're on their own to adjust practice to better serve students. What guidance they get often is plagued by vague teaching standards, overly numerous and often trivial learning objectives, and testing systems that measure only some of the outcomes that educators value for students. Success itself remains ill defined. The partners in the Measures of Effective Teaching (MET) project--a group of thoughtful individuals in districts, unions, schools, research institutions, and technical organizations--told from the outset that current evaluation systems were not being used to improve teacher support. To help states and districts navigate the work of implementing feedback and evaluation systems that support teachers, this paper offers nine guiding principles based on three years' of study, observation, and collaboration with districts. Prior reports tested, and ultimately supported, the claim that measures of teaching effectiveness could be valid and reliable. These principles fall into three overarching imperatives: (1) Measure Effective Teaching; (2) Ensure High-Quality Data; and (3) Invest in Improvement. (Contains 1 figure.)