This thesis consists of two articles discussing the evolving discipline of Enterprise Architecture (EA), along with introduction and discussion of EA, systems approaches and the nature of theory in Information Systems Science. Despite decades of research, diversified views of the definition, scope and purpose of the concept occur, and no common theoretical foundation has been acknowledged. Recently, different systems approaches have been of interest in the field of EA research, both as means of evolving the scope of EA, and by strengthening theoretical foundations of the field. In the first part of the study, the definitions of enterprise architecture, gathered from previous literature and 26 in-depth practitioner interviews, are examined through Lapalme’s Schools of Thought on Enterprise Architecture. The second part of this study contributes with a systematic literature review on the state-of-art of systems approaches in EA research, which have been phrased as a possible theoretical basis for the field. The findings are discussed, reflecting to the types of theory and the use of theory in EA field of research. The results of this study indicate that while there is still no shared definition of EA, its scope and purpose are increasingly extending from IT-business alignment towards holistic organizational design and development in the system-in-environment setting. Although various systems approaches are frequently referred to in the EA studies, the application of systems theories appears to be fragmented, and the approaches are rarely systematically used in empirical studies.