Programs operate on data. It is thus natural to start our considerations of how to think about programs by a discussion of how to think about data types. For this purpose, we do not really need to know how the objects of a type are concretely represented (such representations have been discussed in Chap. chapter:models); we may rather focus on the properties that are satisfied by the operations which have been given to us to work with these objects. This view is also in line with modern software engineering that abstracts from the implementation details of data by encapsulating them in classes that only expose a (more or less) well documented method interface to the user.