Among the many branches of computer science, ubiquitous computing enjoys an unusually distinguished history of creating and deploying prototypes. Why is this? A tempting answer is that many ubicomp researchers and practitioners have backgrounds in subjects such as HCI and systems--areas with a strong focus on learning from deploying working prototypes. However, a more compelling answer might be that ubicomp, unlike almost every other subspecialty with the exception of HCI, is about embedding computing into existing human systems. This introduction is part of a special issue on Real-World Deployments.