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One of the hot topics in ubiquitous computing is context-aware services, which automaticlly adapt user based on user's context information, e.g., location, time, favorite etc. Situation-aware services are advanced context-aware services, such that more complex context information and relations among users, objects, and environment are taken into consideration. In order to provide the situation-aware services, it is necessary to describe the situations, to define and link services which will be provided in the situations, and to detect the happening of the situations in the real world. Though many researches have worked on context-aware services, there are not so many on situation aware services, especially on describing and detecting the situations uniformly. In this paper, we take a composition approach to solve the problems. We first define primitive situations which are based on J. Barwise's situation theory and can be detected by current ubiquitous techniques. And then, we present several manipulation rules, by which more complex situations can be composed. Our methods can be used as a building block in construction situation-aware services, such as warning to a user to prevent him/her from dangerous situations, reminding a user not forgetting to bring belongings with him/her, navigation to a handicapped user, etc.