First of all, I would like to thank three people in my department from whom I have borrowed a great deal of time and knowledge.Susan Pintzuk, Beatrice Santorini, and Ann Taylor did in their dissertations most of the crucial research that made it possible for me to carry out the present investigation.I don't think I could have been able to make much sense of the Old Spanish clitic facts had I not counted with their work.It is still very amazing to me that our research wound up being so related.Honestly, before starting with this, I could never have suspected that I would be obtaining so many useful clues from work on languages such as Old English, Yiddish or Homeric Greek, which had been carried out by three other graduate students in my department.But most of all, besides inspiring me intellectually, they have been good friends and have given me that of which they have very little: their time.Many thanks are also due to my advisor, Tony Kroch, and to the members of my dissertation committee, Sabine Iatridou, and Juan Uriagereka, for their many suggestions and valuable assistance in this dissertation project.Tony introduced me to the study of syntactic change and made this intellectual experience a truly fascinating one.Sabine and Juan had to suffer through all the preliminary drafts of this dissertation, and gave me many insightful comments and helpful advice.Their contribution has definitely lead to a substantial improvement of the final product.There are also many people who have unselfishly given me much of their time.Special thanks to Aaron Halpern, Philip Miller and Dieter Wanner,
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