Diese Dissertation ist auf den Internetseiten der Hochschulbibliothek online verfügbar. I would not have been able to write this thesis without the help of my family. I would like to thank my wife Antje, who gave me love and support; my parents, on whose assistance I could always count throughout the years; my sister, who didn't stop nagging me about finishing this thesis; Marianne, who shared Antje's burden of taking care of our twin daughters Johanna and Annika while I was working overtime. And I would like to thank you, Annika and Johanna, for being my favourite distraction. Without you, writing this thesis would have been much less interesting and challenging. I am also greatly indebted to my colleagues who have contributed to this thesis in numerous ways. Many of the discussion we had have found there way into this thesis. I would like to thank Franz Baader, my thesis advisor, whose encouraging " haeh? " forced me to write better papers; Ulrike Sattler and Carsten Lutz, who never grew too annoyed with my sloppiness; and all my colleagues, who happily endured the constant interruptions when I needed to discuss some new ideas. Thanks for the good time I had while working with you.