The early designers of the Internet fostered tremendous innovation by leaving much of the network’s functionality to the programmable computers at its periphery. Unfortunately, the *inside* of the network has been much harder to change. Yet, changing the network is important to make the Internet more reliable, secure, performant, and cost-effective. The networking research community has struggled for many years to make networks more programmable. What has worked, and what hasn't, and what lessons have we learned along the way? This talk offers my perspective on these questions, through a 25-year retrospective of research on programmable networks, focusing on my own research experiences as well as reflections on major trends in the field. The talk advocates a sort of “ambitious pragmatism” that approaches an ambitious long-term goal (a programmable network infrastructure) through smaller, pragmatic steps while keeping an eye on the prize.