To be prepared for the 21st century, it is critical that all students have sufficient knowledge of and skills in science. Studies suggest that high-quality teaching can make a significant difference in student learning. The National Science Teachers Association (NSTA) believes a high-quality science teacher workforce requires meaningful, ongoing professional development. To achieve this goal, schools and school systems must devote time and resources to effective professional development for "all" K-16 teachers of science and science educators to support learning throughout their careers. This paper discusses the following: (1) principles behind professional development in science education; and (2) considerations for designing professional development in science: planning, implementing, and sustaining. (Contains 8 resources.)