The primary aim of this essay and qualitative case study is to identify different explanations of why the European External Action Service (EEAS) was organised and why it resulted in today’s organisation. Existing research not entirely updated highlighted the need for new information. Furthermore, since existing research mostly focuses on other aspects, such as the character of EU foreign policy or the role of the EU internationally, this motivated an alternative approach. Three main branches of New Institutionalism (rational choice, historical and sociological) constitute a theoretical framework, aimed at identifying explanations perhaps not earlier contemplated. Findings suggest for example that a major reason for the organisation of this service is due to self-interest maximising, increasing the EUs political and economic influence by acting more coherent. The need to attain legitimacy and resemble other established actors by adopting institutionalised practices and structures in the homogeneous diplomatic field is another explanation. The Service is partially organised the way it is because of the successes of major EU institutions and the member states in ‘locking-in’ their preferences. Organisational characteristics can also be explained as results of several historical, institutional upgrades in relation to earlier treaties and debates on the future of Europe.