This chapter studies levels of analysis. Research in the social sciences may be interested in subjects located at different levels of analysis. The level of analysis indicates the position at which social and political phenomena are analysed within a gradual order of abstraction or aggregation that is constructed analytically. Its definition and boundaries vary across social science disciplines. In general, the micro level refers to the individual level and focuses on citizens’ attitudes or politicians’ and diplomats’ behaviour. Analyses at the meso level focus on groups and organizations, like political parties, social movements, and public administrations. The macro level corresponds to structures that are national, social, economic, cultural, or institutional — for example, countries and national or supranational political regimes. The explanandum (what research aims to account for), the explanans (the explanations), the unit of analysis, and data collection can be located at different levels. The chapter then considers two main errors commonly associated with aggregation and levels of analysis: ecological and atomistic fallacies.