Unlike other sub-disciplines of physics, physics education research (PER) studies humans who are dynamic, imprecise, and evolving. In this talk, I review historical work in PER, consider the state of the field, and propose a number of big questions that will shape the future of PER (and physics). Replication, evolution of teaching, diversity and inclusion, and establishing causal mechanisms in learning are among the big questions that will guide the work within Drexel’s PER Network.