Guest Editors' Introduction: Building and Evaluating Ubiquitous System Software

From its inception, plenty of vision has existed in ubiquitous computing. This issue concentrates on how to realize the software infrastructure for that vision: on building and evaluating system software. Ubiquitous computing raises major challenges for system software researchers, mainly because of the heterogeneity and volatility that characterizes it. The set of participating users, hardware, and software in ubiquitous computing environments is highly dynamic and unpredictable. How far have we come in meeting the challenges?