The present text is a slightly revised version of my dissertation, which was accepted at the University of Bremen in May 2009. The work was developed as part of my employment in the research project "Transnationalization of Social Relations," which was funded by the German Research Foundation (DFG) from 2005 to 2008. The completion of this work would not have been possible without the help of many people. I owe a great debt of gratitude: to Steffen Mau for his long-standing support and constructive criticism; to Johannes Huinink for his helpful comments and always open ear; to the DGS working group 'Network Research' for their theoretical and practical insights; to Roger Häussling and Christian Stegbauer for their interest in including my work in the VS series 'Network Research'; and to the IT manager Hartmut Asendorf.