Writing a doctoral thesis is a hard and challenging experience, particularly because it is something that develops over the course of quite a long period of time, in which many personal and professional incidents may inadvertently happen. Especially when you do it at the same time you juggle a working career, as was my case. For both professional and personal reasons, I have many people to thank. First and foremost I have to thank my supervisor, Nuno Sousa Pereira, not only for his supervision but also for his friendship in the course of these years, and very particularly for always believing in me, ever since I met him. For similar reasons, I also have to single out Elvira Silva amongst all of my PhD teachers, for her constant moral support and availability to share her professional opinion in a great deal of matters. I would also like to thank Manuel Mota Freitas Martins, for his support as my tutor for the curricular part of the PhD program. A special thanks goes to all of the people who work with me every day, in my teaching activities, for a lot of the times allowing me to give priority to my PhD schedule. I specially thank Pedro Duarte Silva and Américo Mendes for all the support. This thesis also benefitted considerably from the comments of Roger Backhouse,