Many national policymakers and experts believe that professional development, which teachers often have regarded as wasted time, is potentially an important tool for improving student learning. Some of the current attention to professional development grows out of several studies in recent years that have highlighted the central role that teachers play in student learning. The federal No Child Left Behind Act reflects that recognition. Besides calling upon schools to staff classrooms with "highly qualified" teachers, the law says schools should annually increase the percentages of teachers in their buildings who receive "high quality" professional development. Experts know that programs focused on the academic content that teachers must cover and on how students think about that content are more effective than those that impart more generic teaching techniques. They know that longer-lasting professional development tends to produce better results. They also know that such programs work best when they link to teachers' daily classroom experiences--the tasks their students will have to do, for example, or the texts they will use. To a lesser degree, researchers also have a hunch that it is important for teachers to engage in learning sessions collectively--maybe with other teachers from the same department or grade--so that they can meet later to reflect on what they learned. (Contains 3 charts.)