Because they focus on the quality of instruction, teacher evaluations can be powerful catalysts for teacher and school improvement. But today, the typical teacher evaluation consists of a single, fleeting classroom visit by an administrator untrained in evaluation. Often he or she wields a checklist of classroom conditions and teacher behaviors that don't focus directly on the quality of instruction. Several models of evaluation demonstrate that it's possible to evaluate teachers in much more productive ways. These models use explicit standards to evaluate teachers, are based on multiple measures, and involve multiple evaluators and evaluations. Because comprehensive classroom evaluation systems are more labor-intensive, they are more expensive than principal drive-bys or evaluations based on test scores. But they are an investment worth making because they improve teachers' performance and signal to teachers that they are professionals doing important work. (Contains 2 endnotes.)