Location-based services (LBS) and mobile systems are two key areas of research into making information services more accessible to untrained users. However, the influence of the situation a user is facing while accessing an LBS on a mobile device is often neglected even though it may have a strong impact on the user's abilities and needs as well as on how a service should interact with that person. In this book, we present a model and an implementation to handle situational interactions on spatial topics as well as several adaptation strategies to cope with common problems in real-world applications. The model is designed to incorporate situational factors in spatial reasoning processes at the basic level and to facilitate its use in a wide range of applications. The implementation realizing the model corresponds very closely to the structure of the model, and was put to test in two scenarios: a mobile tourist guide and the mobile component of a multi-modal dialog system. The adaptation strategies address the lack of information, resource restrictions as well as the problem of varying availability and quality of positional information.