As we enter a new millennium, the quality of life in the decades to come will depend to a great extent on the quality of our schools. A hallmark of that quality will be our ability to provide effective science instruction for all students. Our success in achieving that objective will depend on teachers at every level who are well-equipped and well-prepared to teach science.  Staffing our classrooms with teachers who are able to provide quality science instruction, though, will not be easy. It will require commitment and collaboration on the part of teachers, administrators, teacher educators, and many others. Nor can we assume that, once in the classroom, teachers will remain well-qualified and well-prepared. A clear understanding of effective professional development, and continuous implementation based on that understanding, will insure that we have the teachers that we, and our students, will need in the coming century.