This article begins by clarifying the distinction between formative and summative evaluation that was first drawn by Michael Scriven (1967) in an influential essay regarding education evaluation. Scriven supplied his analysis soon after the Elementary and Secondary Education Act (ESEA) was enacted in 1965--a time when almost no serious attention had been given to the nature of education evaluation. Because ESEA provided unprecedented federal dollars to support states' education programs and also required that the programs be evaluated, US educators soon clamored for guidance about how to do this thing called "education evaluation." Scriven distinguished between the two roles of evaluation as they applied to the appraisal of education programs. Formative evaluation was intended to determine the worth--that is, the merit--of not-yet-completed education programs. The role of formative evaluation was to give educators information that could be used to improve still-malleable programs. Summative evaluation, in contrast, was intended to determine the worth of mature, already-completed education programs. The role of summative evaluation was to help decision makers determine whether to continue or terminate a given program. Formative teacher evaluation, then, is focused on improvement; summative teacher evaluation is focused on removal or rewards. The author asserts that both the formative and summative missions of teacher evaluation are tremendously important. He makes the point that if they contaminate each other, however, then the obvious solution is to engage in both of them, but to do so separately. That is, evaluators must distinguish between formative and summative teacher evaluation and keep those two roles apart--totally. The author closes by warning that new, high-stakes teacher evaluation systems emerging all over the United States can do plenty of good, but they can also be harmful. He cautions that inappropriate evaluation could have a disastrous impact on schools. Because he believes that formative and summative teacher evaluation can each make significant contributions to instruction, James Popham champions the idea that both should be implemented widely--but separately-- and thoughtfully as components of emerging teacher-evaluation systems.