I am thankful to my supervisor, Gábor Vattay, whose scientific curiosity introduced me to great challenges as well as to new friends and colleagues. He not only made it possible for me to join a research group abroad but also to return and continue my work as a physicist in my hometown. My professors at ELTE, especially István Csabai, for his guiding vision in the projects I participated in, and to Tamás Geszti, Lajos Diósi, Tamás Tél, Imre Kondor, and Tamás Vicsek. Their inspiring lectures and conversations made this Doctoral Program attractive and unique in its thematic diversity. The French Institute of Budapest and Timur Friedman and his team at UPMC for the opportunity to join their Internet monitoring research. The Ericsson Research team, especially István Gódor, Szabolcs Malomsoky and Zsolt Kenesi for the opportunity to join their urban sensing research with MIT Senseable City Lab and for their continuing support. My friends and collaborators: H Peti, M Peti, Józsi, and Laci: thank you for showing us the way to both independent research and scientific teamwork; Norbi, Dani, Eszter, Janó, and Tomi for creating a fun and stimulating environment. Zsuzsa and Nóra: thanks for the many memorable moments.