If standards-based, performance-based assessments are to fulfill their promise of driving improvements in student learning and achievement, assessment systems must incorporate the means for affecting what teachers do and how they think about what they do in their classrooms. There is a growing body of evidence that teachers learn and change through processes of "practical inquiry" whereby teachers examine, reflect on, make sense of and, when relevant, choose to change their own practice in the light of what their inquiry has revealed. In this paper we propose that, for an assessment system to have positive impact on teaching and learning, teachers need to use assessment as a framework for practical inquiry about their students' learning. We investigated this hypothesis in the 1992-93 school year in a project o to develop curriculum-embedded assessments for the California Learning Assessment System (CLAS). Four teams of California teachers, in English-language arts, history-social science, mathematics, and science, developed, tested in their own classrooms, and then reflected on and revised assessment tasks. They were provided social support, as well as a conceptual framework that stressed thinking about evidence of student performance. Most of the teachers in a sample of 50 reported changes as a result of this experience. Teachers reported assessment-related changes, such as decreasing reliance on end-of-unit tests in favor of other sources of evidence of student learning, as well as instructional changes, such as revising and refining their instructional goals and sharing responsibility for learning and assessment with their students. It appears, then, that when teachers, in collaboration with colleagues, spend time thinking about, looking for and evaluating evidence of student learning in the context of activities that take place in their classrooms, there is a significant opportunity for them to grow. We conclude that, although what is needed to foster such change is not trivial, it may be possible to alter some current forms of teacher involvement in assessment-related activities or to combine teacher development with assessment-related activities in ways that are both cost-effective and beneficial.