At that point, I have to express my thankfulness to my friend and "fellow traveler", Giorgos Gkikas. Studying together for those 9 years has been an honor for me, as he is an eminent researcher and, in parallel, a perfect character. I would like to thank him for his full support and for our great collaboration.The next person I wish to acknowledge is a "special" person in my life, Orsa Boura.However, it would be a shame of me if I restricted my gratitude to her in just a paragraph.Briefly to praise her brilliant character and smart personality, along with her sea breezeattitude and mentality.Last but not least, I wish to thank my family members, my parents Athanasios and Vagia as well as my brother Evangelos. They have been helping me and supporting me in many ways, especially during these 9 years of my studies. They kept providing me all the psychological and financial support in order to achieve the maximum in my life.Τέλος, ένα πολύ μεγάλο ευχαριστώ στην οικογένεια μου, στους γονείς μου Αθανάσιο και Βάγια και στον αδερφό μου Ευάγγελο, που με την αδιάλειπτη συμπαράσταση, υποστήριξη και κατανόηση τους σε ηθικό αλλά και υλικό επίπεδο μου δίνουν δύναμη και πυγμή ώστε να ανταπεξέρχομαι και να αποδίδω τα μέγιστα σε ότι πραγματεύομαι.