With the advent of portable devices (e.g., smartphones) and the advances in wireless networking technologies (e.g., WLAN, GPRS, UMTS), the vision of ubiquitous computing is becoming a reality. It aims to facilitate user tasks through the seamless utilization of heterogeneous computing and communication capabilities (represented as services) available in the environment. Service discovery, which is necessary for achieving the above goal, must be aware of the service's non-functional properties due to the challenges posed by ubiquitous computing, such as device portability and mobility. This thesis proposes an overall solution that supports QoS-aware service discovery in ubiquitous computing environments. Our contribution lies in substantiating QoS awareness in the following three aspects. Firstly, during the process of discovering services, the expiring wireless links resulting from device mobility are identified and avoided since they cause service failures and thus hamper service reliability. Secondly, as mu tiple services can be discovered, a comprehensive utility function is proposed to evaluate services in terms of their various non-functional properties, meanwhile taking into account the service user's preferences among them, for the purpose of selecting the best one. Thirdly, to avoid untrustworthy services, a distributed reputation mechanism is proposed to facilitate the evaluation of the service host's trustworthiness. The above three proposed solutions are extensively evaluated respectively, based on analysis and simulation. They are further incorporated into a middleware that supports QoS aware Web service discovery in ubiquitous computing environments. A prototype implementing the middleware is deployed and evaluated. The results show that the overhead introduced by QoS awareness seems reasonable.