Previous approaches to teacher testing have been criticized for poorly representing the knowledge base for teaching, for oversimplifying teaching decisions, and for lacking criterion-related validity evidence supporting their use. A new generation of teacher assessments has been developed in the United States through the efforts of the National Board for Professional Teaching Standards and a corollary organization of more than 30 states. These performance-based assessments use videotapes of teachers' practice, examples of lessons and assessments, samples of student work, and analyses of classroom events and outcomes to provide evidence about teaching. Early research on the effects of these assessments suggests that they may be more valid measures of teacher knowledge and skill and that they may help teachers improve their practice. The stimulus to teacher learning appears to occur through task structures that require teachers to learn new content and teaching strategies as part of their demonstration of performance and through the processes of required re¯ection about the relationships between learning and teaching.As a knowledge-based economy emerges, few doubt that many more students need to be educated to much higher standards and that, as a consequence, teaching is increasingly challenging work. Many educators and policy-makers also agree that the capacities teachers need in order to succeed at teaching much more challenging content to a much more diverse group of learners can only be widely acquired throughout the teaching force by greater investments in teacher preparation and development. Such reforms, a number of recent reports have argued, will require comprehensive restructuring of the systems by which states and school districts license, hire, induct, and support teachers, and provide for their continual learning