Teacher evaluation has evolved from a task used for administrative decisions to an activity whose main goal is the enhancement of student learning and well-being through the improvement of instruction. The actual implementation of teacher evaluation by school principals will determine greatly the extent to which it can achieve this goal. An attempt was made to examine how principals' implementation of teacher evaluation was related to their reasons for evaluating (formative or summative), their perceptions of its benefits, their preparation for the evaluator role, and several background variables. Data were gathered by questionnaire from 219 school principals in Israel. Findings indicated that evaluating for improvement rather than for administrative reasons, believing teacher evaluation to benefit school functioning, and feeling adequately trained for the task significantly predicted fuller implementation of the teacher evaluation model. Implications of the findings for preparing and supporting school principals in their role as evaluators is discussed.