Ontology and the related term "semantics" have recently found increased attention in database discussions. Early discussions of ontology issues important for databases [126,78] were lost in a sea of papers on technical, mostly performance issues, despite the fact that textbooks as early as [134] discussed briefly the relationship between information system and real world. This is different today; interest in semantics has increased, and this will be more so in the future given the current interest in the Semantic Web [11,10] (see Chapter 9 of the book for related discussion).Information systems and their implementation as databases rest on ontological commitments. Decisions about the type system used, how identifiers are managed, and so on, are derived from a specific view of the world to which the database relates, in other words from a specific ontology. The ontologies of standard database models make very limited assumptions and therefore the data model is widely applicable. Spatio-temporal databases must make stronger commitments to capture the meaning of space and time. Such an ontology is necessarily more involved and the connection to the application area stronger. The designer of a database application has to reconcile the ontological concepts from the application area with the ontology built into the database. Optimally, a spatio-temporal database involves in its built-in ontology a minimal commitment on how space and time is structured and is thus most open for application specific refinements. Exploring the minimal set of ontological commitment is the goal of this chapter.The ontology built into a DBMS can be insufficient or it can be too restraining. It is insufficient if the ontological categories necessary for numerous applications are not available and must be reconstructed for each application anew; the resulting incompatibilities will be very costly to correct later [81]. It is too restraining if the ontology commits those who apply it to assumptions which do not hold for novel applications. Spatio-temporal databases are typically constructed to integrate the knowledge of many agents and face the problem of heterogeneous environments, a point already raised by Wiederhold et al. [190, Chapter 22]. Current databases do not allow us to model joint beliefs of groups of agents which do not correspond to similar beliefs of other groups of agents; for example, Reuter works with groups of scientists, who manage terabytes of reports of results from experiments in cellular biology, where the validity of the results and their interpretation are debated among the groups. Current ontological investigations related to databases and information systems have been