This book summarizes the main topics related to the understanding of the European Union focusing on: the history of the development of the European Union, the main decision-making institutions, the way of functioning, enlargement and relations with the Balkans and Kosovo. The book is dedicated to high school students and addresses topics that are very superficially included in the school curriculum. This book first reflects the history of the formation of the European Union, from the first ideas for the formation of the EU, the signing of the treaties, the historical moments that have deepened integration, and the various challenges that the member states of the European Union have gone through. In addition to the history of the EU, the following chapters deal with the institutional architecture of the European Union under the Lisbon Treaty, which is in force. The most important decision-making institutions will be treated separately describing the manner of election, composition, role, duties and responsibilities of each institution. These chapters also include decision-making modalities as well as institutional interdependence in the decision-making process within the EU. Finally, this book addresses the enlargement policy of the European Union, focusing on the relationship of the European Union with the Western Balkans, namely Kosovo. This book is based on official data of the EU institutions and Kosovo institutions and the attached literature. Considering the readers to whom this book is addressed, the description is simplified and the number of citations is reduced.