Enterprise architecture originates from the 1980’s. It emerged among ICT practitioners to solve complex problems related to information systems. Currently EA is also utilised to solve business problems, although the focus is still in ICT and its alignment with business. EA can be defined as a description of the current and future states of the enterprise, and as a change between these states to meet stakeholder’s goals. Despite its popularity and 30 years of age, the literature review conducted on top information and management science journals revealed that EA is still lacking the sound theoretical foundation. In this conceptual paper, we propose General Systems Theory (GST) for underpinning theory of EA. GST allows us to see enterprises as systems of systems consisting of, for instance, social organisations, humans, information systems and computers. This explains why EA can be used to describe the enterprise and its components, and how to control them to execute the managed change. Implications to science and practice, and some directions for future research are also provided.