This thesis dealt with the question how preadolescents entering ethnically heterogeneous secondary schools make decisions about peer-group relationships. The overarching question, how cross-ethnic friendships form, was dealt with by examining: (1) the trajectory of friendship choices in newly formed school classes over the course of one school year; (2) the role of several key predictors of preference for same-ethnic friends (PSF), namely outgroup orientation, intergroup attitudes, peer norms, contact conditions, and shared identity; (3) the role of ethnic group membership in determining differences in friendship choices and in predicting PSF; (4) the stability of same- and cross-ethnic friendships; and (5) the role of empathy and peer norms in predicting the stability of cross-ethnic friendships. The main findings were: First, Turkish preadolescents showed marked PSF at the beginning of the year while German preadolescents did not. Second, the trajectories of PSF were different for German and Turkish preadolescents. While German preadolescents showed a curvilinear trend, Turkish preadolescents showed a linear downward trend. Importantly, both groups did not show significant PSF at the end of the year anymore. Further individual variation in PSF was explained by the predictor variables. Specifically, outgroup orientation, intergroup attitudes, peer norms, and perceived contact conditions helped explain observed variability among German preadolescents. Among Turkish preadolescents, intergroup attitudes, peer norms, and the time-varying effect of class identification explained variance in PSF. The analysis of friendship stability revealed that, as predicted, cross-ethnic friendships were less stable than same-ethnic friendships. It was further shown that for German but not Turkish preadolescents, empathy and positive peer norms about cross-ethnic friendships had a facilitating role in maintaining cross-ethnic friendships over a five-month-period.