States, districts, and schools all across the United States are busy developing or implementing teacher evaluation systems. One can trace this flurry of activity to a variety of reports and initiatives that highlight two failings of past efforts: (1) Teacher evaluation systems have not accurately measured teacher quality because they've failed to do a good job of discriminating between effective and ineffective teachers; and (2) teacher evaluation systems have not aided in developing a highly skilled teacher workforce. Although efforts to move quickly in designing and implementing more effective teacher evaluation systems are laudable, it is important to acknowledge a crucial issue--that "measuring" teachers and "developing" teachers are different purposes with different implications. An evaluation system designed primarily for measurement will look quite different from a system designed primarily for development. Both measurement and development are important aspects of teacher evaluation. When measurement is the primary purpose, a small set of elements is sufficient to determine a teacher's skill in the classroom. However, if the emphasis is on teacher development, the model needs to be both comprehensive and specific and focus on the teacher's growth in various instructional strategies. These distinctions are crucial to the effective design and implementation of current and future teacher evaluation systems. (Contains 1 figure.)