Today, science teachers continue to seek ways to improve their instruction and become credentialed as "highly qualified" in their field. This paper describes how science and science education faculty at the University of Massachusetts and veteran K-12 science teachers, with funding from the National Science Foundation, designed Science Education Online (SEO)--an online, graduate-level program for elementary and middle school science teachers that fosters learning of science and science teaching. The SEO program consists of 2 courses in educational pedagogy and 12 content-based courses. Through data collected in the form of course evaluations, student surveys, and interviews, teachers described how the hands-on, inquiry-based activities, interaction with their peers and instructors, and links to classroom practice embedded in each of the online courses enabled them to increase their subject matter knowledge and understandings of effective teaching. Data indicate that SEO courses meet the university's standards for high-quality instruction and purposeful learning opportunities, and facilitate teachers' acquisition of needed, advanced credentials. (Contains 7 tables and 4 figures.)