In this dissertation on "Lisbon Treaty and the European Union" we will talk about the Treaty of Lisbon as one of the Founding Treaties of the European Union, the treaty last adopted at the level of the European Union, the treaty under which the European Union exists in its current form and which is currently in force. After the basic theses on the European Union (definition and some general terms), in the first part of the paper we will see how the Treaty of Lisbon came about, and which of the founding treaties preceded the one currently in force. In the elaboration of the paper, we will go through all the most important aspects of the Treaty, and how much the European Union has actually changed in order to reach the state it is today, ie according to the provisions of the Treaty of Lisbon. We will see that the road to the Treaty of Lisbon was not easy at all, it was thorny, full of obstacles and protests from some members, especially the failed attempt of the European Constitution which in some parts of Europe even led to chaos. The fact that the adoption of the Treaty was not simple or painless is sufficiently indicated by the fact that, after being confirmed and ratified by the Parliaments of all 27 signatory Member States, it entered into force only two years later, on 1 December 2009. We will have a small window into how the EU looks and works today and what the Treaty of Lisbon has brought to the European Union, in a positive or negative sense.