Context-aware pervasive systems are emerging as an important class of applications. Such work attempts to recognize the situations of entities. This position paper notes three points when modelling situations: (1) there can be multiple ways to represent a situation; (2) a situation can be viewed as comprising relations between objects and so recognizing a situation boils down to determining if a prescribed set of such relations hold or not hold at that given point in time; and (3) situations can be represented in a modular manner, emphasizing an incremental approach and reuse when building a knowledge base of situations. We also present a declarative approach to building contextaware pervasive systems, where relations that are in a situation can be recognized.