Writing a book is not possible without the generous input of many people. It is a pleasure to have the opportunity to thank at least some of these people. Prof. Dr. Jochen Michaelis, the supervisor of my dissertation, taught me how to do economic analysis and initiated my interest in labour market is­ sues. Discussions with him have always been enlightening and have greatly improved the analysis in this book. Moreover, he always encouraged me when I experienced a slump in my motivation. He never lost his calmness and good temper, not even in situations when my need for discussion must have been bothering him. Thanks for that Jochen. I'm indebted to Prof. Dr. Peter Weise for taking over the job as the sec­ ond referee of my thesis. He gave very valuable comments and sacrificed his christmas holiday to write the referee report as fast as possible. I also want to thank Prof. Stefan Voigt and Prof. Dr. Reinhold Kosfeld, the other two members of the dissertation committee, for the discussion during the defence of the thesis.