Acknowledgements Everything started one day of June 2000, when I decided to go Erasmus at the Ecole Polytechnique in Paris, and I was suggested to attend there the databases lectures given by Prof. Abiteboul. When in January 2001, I decided to follow the extra-lectures of Prof. Abiteboul in order to present a databases project, he was so nice to sit beside me and teach me how to write my first HTML page: my homepage. This was his way to introduce me to XML! One first internship at I.N.R.I.A. was my first great experience with research, and from that day on, I never gave up " dreaming research ". When I came back home, I met Maurizio (whose lectures were the most exciting I ever had) and thank to him and Serge I could participate to VLDB as a volunteer (Rome, Sept. 2001). How could someone resist loving research after such a wonderful conference? Then, I finished my exams and Maurizio supported me to come back to I.N.R.I.A. for a second internship (my final project which lead to my graduation thesis). On my return, I started collaborating with Maurizio, and he made me love databases theory and data integration issues so much, that I chose to start my PhD route... Thanks to an European initiative, and thanks to both my advisors who had to fight against italian and french bureaucracy, I had the opportunity to do my research in joint work between the roman and the parisian databases groups. This was not always so easy... But I was so lucky to find such great researchers, both able to have such an amazing " big picture " ! They were both mentors, fathers and friends. No word may express how much I would like to thank you both, Maurizio and Serge. I can only say once again: " Grazie-Merci " (as I am now used to concluding all my research talks). I will miss so much being such a favoured PhD student! Of course, these aknowlegements cannot end without thanking my sweet husband Mario, and my family. Both have been so patient, understanding... and, above all, they have always been " with " me. I love you, and will always do.