Analyzed in the article are the key trends of ethnopolitical and ethnosocial development in the largest post-Soviet states - Russia, Belarus, Kazakhstan and Ukraine. The criteria and foundations of such comparative analysis are formulated and the main problems of nation building in post-Soviet states are shown. Special attention is paid to the relationship between ethnic and civic components of identity, as well as to the identity policy in every state in question. It is mentioned that the formation of civic identity and civil society in post-Soviet countries is a very difficult process. To a large extent these difficulties are caused by the unfinished processes of nation-building and by the ineffective political and economic institutions. It is demonstrated that situation in the field of interethnic relations and in the sphere of national identity is quite different in Russia, Belarus, Kazakhstan and Ukraine. The consolidation of polyethnic nation and the formation of civic identity in Russian Federation is a rather slow process; however, the country possesses important long-term traditions of identity policy and interethnic conflicts regulation. In Belarus, the Soviet identity dominated in the 1990's and the 2000's, while nowadays the new generation wishes to form its own national identity. President Nazarbayev in Kazakhstan tries to shape a complex identity in order to stabilize the social and political situation. This complex identity includes ethnic, national and supranational (so-called Eurasian) components. It is concluded that the experience of formation of such complex identity is useful for Russia and other post-Soviet states. Just the opposite, the social and cultural situation in Ukraine is very difficult: the country undergoes a serious social and ethnic polarization, which invokes political confrontation. These processes result in the crisis of Ukrainian statehood and in different social, political and interethnic conflicts.