This national study of 1858 teachers participating in professional development(PD) summer programs in the United States encompasses a wide range of science subjects and grades. It was conducted to identify program features associated with increases in two types of teacher knowledge that are goals for most professional development programs: teacher subject matter knowledge and knowledge of student misconceptions. The frequency of occurrence of 25 common program features was estimated by participants in 231 different PD programs and used to model teacher gains on assessments keyed to the national science standards by grade-band and science subject, while controlling for teacher background. Surprisingly, gains in subject matter knowledge were small when aggregated across all programs (d = 0.06), with no gains observed in knowledge of student misconceptions, on average. Only a single program feature, focusing on foundational science concepts, was positively associated with gains in both types of teacher knowledge measured. Many popular program features were not associated with increases in teacher knowledge, including: developing original curricula / activities, learning the newest scientific thinking on a topic, conducting scientific research, observing and critiquing classroom instruction, nor designing assessments.