Database management systems are highly specialized to efficiently organize and process huge amounts of data in a transactional manner. During the last years, however, database management systems have been evolving as a central hub for the integration of mostly heterogeneous and autonomous data sources to provide homogenized data access. The next step in pushing database technology forward to play the role of an information marketplace is to actively notify registered users about incoming messages or changes in the underlying data set. Therefore, notification services may be seen as a generic term for subscription systems or, more general, data stream systems which both enable processing of standing queries over transient data. This article gives a comprehensive introduction into the context of notification services by outlining their differences to the classical query/response-based communication pattern, it illustrates potential application areas, and it discusses requirements addressing the underlying data management support. In more depth, this article describes the core concepts of the PubScribe project thereby choosing three different perspectives. From a first perspective, the subscription process and its mapping onto the primitive publish/subscribe communication pattern is explained. The second part focuses on a hybrid subscription data model by describing the basic constructs from a structural as well as an operational point of view. Finally, the PubScribe notification service project is characterized by a storage and processing model based on relational database technology. To summarize, this contribution introduces the idea of notification services from an application point of view by inverting the database approach and dealing with persistent queries and transient data. Moreover, the article provides an insight into database technology, which must be exploited and adopted to provide a solid base for a scalable notification infrastructure, using the PubScribe project as an example.