I would like to dedicate this thesis to Hänk Angerman who introduced me to the world of mathematics at the age of twelve. I will not completely blame him for me going into mathematics but he was a great stimulant. Then there is many people how I would like to pay some gratitude. First of all there is Dick de Jongh who was able to support me during all that period when today was mañana but also when mañana all of a sudden was today. I think I have been supervised very well and learned a lot during the production of my thesis. Also Albert Visser has been a great support. The sessions with Frank Veltman, Dick de Jongh and Rosalie Iemhoff proved to be very fruitful. I would like to direct some special word of gratitude to Carlos Areces with whom I shared a room at the department for some time. His many contributions have speeded up enormously the writing of this paper. Also some word of thanks for Johan van Benthem and Anne Kaldewaij with whom I collaborated on giving the first year's course on Logic and Programming. Via this course I got the idea of using invariants while I got payed at the mean time. The ILLC has been very generous to me with all their very useful facilities. (I even got a Christmas box!) Y gracias también a Emma por el apoyo y el cariño durante el desarollo de esta tesis. Then I would like to thank Rosalie and many other people as well. Probably I forget half of the people I should mention. Sorry for that folks. Finally a word of gratitude to my mother not only for her support the last months but also for having introduced me to this wonderful world in which the best of all worlds is at least interpretable.