Acknowledgments

I wish to thank all who have helped me during the preparation of this PhD. More specifically, I would like to thank my parents who supported me when I really needed it. I would like to thank my examinators, Marco Tomassini and Dave Cliff, for reading and commenting on my work. I would then like to thank Michèle Courant and my research group for providing me with an environment in which I was able to lead my research very freely.

During the years I spent in Fribourg, a number of other people have also brought their more personal contribution to this work. I would like to mention Alban Soupper who has been a great help, first as a student, then as a collegue and always as a friend. Lukas Theiler and Simon Schubiger for always being ready to lend a hand or have some fun. Fabrice Chantemargue for sharing his energy in work. Patricio Lerena and Frederic Bapst for being symbols of ``scientific curiosity''. Rolf Ingold for always giving me good counsel in many matters. And last but not least our secretary team, in chronological order: Monique, Sylvie, Marie-Claire and Yvette, who were always open for a good laugh and a healthy bit of criticism.