Enlargements of the European Union (EU) and the North Atlantic Treaty Organization (NATO) present major puzzles. While many post-communist countries, such as Poland, the Czech Republic, Romania, Bulgaria, Bosnia, Albania, and the Baltic States, were granted a membership or a prospect of membership in these organizations, a number of post-communist countries located in Europe, such as Russia, Belarus, Moldova, and Azerbaijan, were not offered prospects of membership even when they would satisfy official criteria for joining the EU and NATO. Some countries, such as Ukraine and Georgia, were officially recognized as potential members of NATO but not the European Union. The question is which factors account for such significant variation in the accession to the EU and NATO among European post-communist countries. Previous studies explained the exclusion of certain post-communist countries by their failure to meet such formal and informal EU membership criteria as liberal democracy, European location and identity, Western Christian religious tradition, high level of economic development, relatively small population size, and popular and government support in prospective members and most influential existing members. Similarly, NATO accession studies emphasized such criteria as democracy, peaceful resolution of internal ethnic conflicts or territorial disputes, and ability to meet NATO military standards. This paper uses comparative and multiple regression analysis to determine which factors affect the European Union and NATO membership of 25 European post-communist countries. The EU accession index and the NATO accession index are derived from the status of each country as a member, as a candidate, as a potential candidate, and as a non-member/not a potential candidate. Such factors, as the level of democracy, the economic level of development, religion, post-Soviet region, population size, violent conflicts, and public support for EU or NATO membership, are analyzed. The study shows that the level of democracy and the economic development level have positive effects on the EU accession, while being a post-Soviet country has a negative effect. Similarly, the level of democracy positively affects the NATO accession, while post-Soviet states have negative likelihood of admission into NATO when all other factors are held constant.