This article studies the process of nation-building in Indonesia. Using a historical approach for the analysis of what is portrayed as a nonlinear, long-term process, it discusses relevant developments during the pre-colonial, colonial and post-colonial eras, with particular attention to the New Order and most recent periods. The analysis focuses on the complex relations between unity and diversity and highlights the multiplicity of frame-works within which inhabitants of the present Republic of Indonesia have constituted their identities, including national, transnational and subnational ones. Two questions that receive particular attention are the role of religion and the relations between the centre and various parts of the country. The article argues that various factors, including religion and ethnicity, have contributed to nation-building in specific circumstances, but have had contrary effects under other conditions. It also shows that progress and regression in nation-building has partially been the voluntary or involuntary effect of the tactical use governments and other political actors have made of manifold communal differences. It adds that the identity of Indonesian citizens becomes increasingly complex and trans- as well as subnational components increasingly important, but that this does not automatically imply the end of the nation-building process.