Object orientation is a leading paradigm in programming languages, knowledge representation, modelling and, more recently, also in databases. The basic idea of object orientation is that the attributes and operations of an object should be associated with it. The interaction with the object is made possible by the public attributes specified by the class which the object is an instance of and by its public operations. The implementation of an operation is encapsulated in the class of the object and can access its private state. This structure allows programs to fulfill the data abstraction principle: the public attributes and operations are the only possibility to manipulate an object and their implementation is not visible to the other objects manipulating it; thus, the implementation of the class can be changed without changing the interaction capabilities of its instances.