This chapter begins with terminology and definitions that are essential for understanding dyadic and group data. Investigation of dyadic or group processes requires that social psychologists use research designs and analysis strategies that recognize the interdependence of social behavior. It discusses a statistical technique that has become indispensable for the analysis of such data, multilevel modeling (MLM). The chapter provides a detailed discussion of dyadic data structures in which both members are measured on the same set of variables. For dyadic data, the discussion centers on the very popular Actor-Partner Interdependence Model (APIM). The chapter considers three topics in the analysis of group data: the extension of the APIM to groups, group studies with a dyadic outcome, and intergroup studies. It also considers the analysis of these three types of measures, with special emphasis on the different sorts of nonindependence in the data.