AcknowledgmentsThis thesis could never have been written without the help of a great many people.First and foremost, I am eternally grateful to my advisor, Professor Gyula Pap, who has been an extraordinary mentor to me throughout the years, and even dealt with my occasional displays of stereotypical graduate-student behavior gracefully. He has constantly opened up new avenues of research (many of which are still unexplored), and accompanied me every step of the way. A glance at the references at the end of this thesis will convince the reader about the results of the school he has built over the years, which I am proud to be a student of.Mátyás Barczy has been a very supportive co-author, and the incredible keenness of his eye has uncovered numerous mistakes, both large and small, in my manuscripts. The Bolyai Institute has been an outstanding place for study, and for starting my scientific career; I have received nothing but kind assistance from every teacher and colleague I had there.In particular, Gábor Szűcs has taught me most of what I know about the foundations of probability theory, and through his example in the teaching and practice of mathematics he has influenced me more in these matters than almost anyone else. I also owe thanks to my I cannot imagine ever coming this far without my extended family. They have stood with me through the good times and the bad, and have always provided me with a stable foundation, which is essential in completing any enterprise. I am, and will always be, deeply indebted to my parents, who first kindled my intellectual curiosity, to my sister, and especially to my lovely wife, who has witnessed firsthand the emotional hardships that were unavoidable in the process of putting this thesis together, had to relieve me of my parental duties when deadlines were around the corner, and yet never wavered in her support. My dear sons have successfully prevented me from sleeping through late-night flashes of intuition, and cheered me up when the writing became too difficult.I am blessed to have so many good people standing by me. One way or another, big or small, they all had a hand in this thesis, except for the errors -those are mine alone.