There is a growing acknowledgement that STEM education, at all levels, is not producing learners with a deep understanding of core disciplinary concepts. A number of efforts in STEM education reform have focused on the development of "student-centered" active learning environments, which, while believed to be more effective, have yet to be widely adopted. What has not been nearly as carefully considered, however, is the role of the curriculum itself as perhaps the most persistent obstacle to effective science education. It is now time to examine not only how teachers teach, but also what they teach and how it affects student learning. Scientific understanding is difficult to master. It took humanity, as a community, many millions of minds, and thousands of years to formulate and establish foundational scientific concepts. An effective curriculum must develop core scientific ideas in a coherent and understandable way. This requires a different approach to curriculum development than the current piecemeal approach where chunks of seemingly disconnected ideas are presented in separate "chapters" or courses, often without explicitly connecting ideas that in fact depend upon one another. While the goal of science education is not for students to achieve "disciplinary expertise," it is (or should be) to help students attain a robust understanding of core ideas. The goal of teachers, and course and curriculum designers, is to plot an effective conceptual progression from where a student is, to where they would like them to end up. When designing a course, they need to think seriously, of what it is they expect students to master; if it is not within their "zone of proximal development," they must alter the curriculum to give them a path to mastery.