The 20 years of both Lithuania’s and Poland’s membership in the European Union provides a special occasion to do some summing up of this period and to take into account not only factual aspects of the European integration process and integration through law, but also its special dimension, which is legal integration. The history of Poland and Lithuania may be used symbolically to serve as an example of widely understood integration processes occurring in Europe. Both these countries have experienced multiculturalism, multiple religions, tolerance, and the building and modelling of national identity. Three historical events had a significant importance in their history and integration process: The Act of the Union of Lublin, declared on 1 July 1569, and in recent history the Treaty between the Republic of Poland and the Republic of Lithuania on friendly relations and good-neighbourly cooperation, signed in Vilnius on 26 April 1994, and the Treaty of Accession of Poland and Lithuania to the European Union, signed in Athens on 16 April 2003. These events and an analysis of documents signed then will allow us to shed light on the on-going processes in Central and Eastern Europe and the common efforts to become EU members. It will also allow an assessment of the essential aspect of integration, namely integration that is taking place in the legal realm. The commitments made and historic moments marked by treaties (such as the three legal acts referred to above) makes this assessment comprehensive and very much desired from the perspective of the activity of both these countries within EU structures; an activity which is celebrating its 20th anniversary this year.