During the past 20 years, the focus of physics education research has shifted several times. In the early 1980s, PER was largely concerned with alternative conceptions that students held about various physics ideas. The mid‐1980s saw an explosion of interest in computer‐based and computer‐aided instruction. In the early 1990s, the PER community published several research‐based curricula that promoted activity‐based or inquiry‐based learning, and throughout the decade the community was increasingly focused on student cognition. As a result of the past two decades of study, physics education researchers know a great deal about how students conceptualize physics and what makes for effective instruction. What is missing is a debate at an international level on the selection of curricular content. In other words, what physics should we teach?