The background of the project is based on the notion of ubiquitous computing. Ubiquitous computing was introduced as a prospective view about future usage of computers. Smaller and cheaper computer chips will enable us to embed computing ability into any appliances. Along with the convenience brought by ubiquitous computing, its inherent features also exposed its weaknesses. It makes things too easy for a malicious user to spy on others. An Intrusion Detection System (IDS) is a tool used to protect computer resources against malicious activities. Existing IDSs have several weaknesses that hinder their direct application to ubiquitous networks. These shortcomings are caused by their lack of considerations about the heterogeneity, flexibility and resource constraints of ubiquitous networks. Thus the evolution towards ubiquitous computing demands a new generation of resource-efficient IDSs to provide sufficient protections against malicious activities. SUIDS is the first intrusion detection system proposed for ubiquitous computing environments. It keeps the special requirements of ubiquitous computing in mind throughout its design and implementation. SUIDS adopts a layered and distributed system architecture, a novel user-centric design and service-oriented detection method, a new resource-sensitive scheme, including protocols and strategies, and a novel hybrid metric based algorithm. These novel methods and techniques used in SUIDS set a new direction for future research and development. As the experiment results demonstrated, SUIDS is able to provide a robust and resource-efficient protection for ubiquitous computing networks. It ensures the feasibility of intrusion detection in ubiquitous computing environments.