Online education is a rapidly growing phenomenon for science teachers. Using a sample of 40 online science courses for teachers offered during the 2004-2005 academic year, the Learning Science Online (LSO) study explores what characteristics of online science courses are most strongly associated with positive learning outcomes among science teachers, after accounting for teachers' prior science experiences and demographics. This research is unique in that it is the first aggregate study of teachers learning science online in a wide variety of educational programs. Hierarchical linear modeling points to changing roles of instructors and students in online courses, with lower perceived levels of instructor support and a supportive course design strongly associated with positive learning outcomes.