National science assessments and international comparisons indicate that science achievement in the United States is stagnant or declining. Of the many steps needed to improve U.S. science education, the authors write, none is more important than improving teacher training and preparation. Analyses of scores on the National Assessment of Educational Progress in science indicate that students score higher when their teachers have had substantial professional development in laboratory skills, hands-on learning, instructional technology, and assessment. The article describes a professional development program focused on these skills--Columbia University's Summer Research Program for Secondary School Science Teachers. The program gives participating teachers real-world laboratory experience and extensive support in taking their expertise back to their classrooms. Evaluations of the program and of another summer program sponsored by the Alabama Department of Education show that such professional development experiences can raise student achievement.