Many readers of this journal are probably familiar with calls from governmental, business, and educational authorities to expand and improve the preparation of science teachers, with a particular focus on the shortage of highly qualified physics teachers. It may seem as if this problem has been around forever, and in fact similar expressions of alarm have been heard for well over a century. Why, then, does this shortage persist? Has the physics community been negligent in offering possible solutions? In fact, the opposite is true: physics educators long ago arrived at a consensus and pointed to a way forward, with a consistent set of recommendations. By tracing the history and elucidating those recommendations, we hope to help motivate physics educators to promote these goals more clearly, and with greater specificity and urgency.