For many educators, professional development brings to mind workshops, night courses at a college of education, or perhaps meeting with colleagues for lesson study. This article discusses teacher externships which offer a very different kind of professional development experience, one that is often transformative for educators and their students. Externships--spending time in a workplace outside the classroom to learn how a discipline is applied in a career setting--"provide a peer-to-peer learning environment." Externships range from a day of simple job shadowing to full externships that are usually project-based and last as long as two weeks or a full summer.