The foundations of the discipline of Enterprise Engineering (EE), as envisioned by the CIAO! Network, consist of the CIAO! Paradigm and a number of theories. After the discussion of the paradigm, which has its origins in the communication-centric view on information systems engineering that emerged around 2000, the role of the EE theories and their relationships with the EE methods and the practice of EE is explained. After having been arranged in a suitable classification scheme, each of the theories is briefly discussed: the EE information theory, the EE model theory, the EE function-construction theory, the EE organisational operation theory, the EE system theory, the EE organisational construction theory, the EE organisational essence theory, the EE design theory, the EE implementation theory, the EE automation theory and the EE governance and management theory.