The Schengen Agreement is widely regarded as one of the greatest achievements of internal integration by the European Union. Free movement of people, goods, and capital in the Schengen Area is a great experiment in the whole history of human kind. However, not all of the European Union Member States are involved, or totally involved in this process. Some countries like the UK and Ireland chose to opt out. Other countries which newly joined the EU like Romania, Bulgaria, and Croatia are still striving to meet all the conditions. Even within the Schengen states, old states and new states have different authorities in several aspects such as visa policies. The Schengen Agreement is helping the European integration. However, different states have chosen different relationships with it and caused three categories of states: associated Schengen State, new Schengen State, and semi-Schengen State. Therefore, Schengen is an “imperfect integration” caused by differentiation. The purpose of Schengen is to unite European countries. However, it alienated some countries from others.