People engage in a multitude of different relationships. They maintain personal relationships with family members, romantic partners, friends, colleagues, neighbors, and other persons from various surroundings. What is the function of different kinds of similarity in these diverse relationships? Similarity in close others is studied within the scope of kin recognition and attraction-similarity theories. It also plays an important role for cooperative behavior. The role of psychological (e.g. skills, global appraisal) and social (e.g. gender, familial status) similarities in connection with emotional closeness and reciprocity of support among relationship persons was examined in four independent studies. In all, results combine evolutionary and social psychological perspectives on similarity in personal relationships and extend previous findings by means of a network approach, multilevel modelling, and experimental manipulations of existing relationships. The findings further show that psychological and social similarity have different implications for the study of personal relationships depending on the phase in the developmental process of relationships.