Guido Caldarelli is currently Associate Professor in the Institute of Complex Systems in the Department of Physics of the University of Rome "Sapienza" Italy. The institute is part National Research Council (CNR) of Italy. He got his degree in physics in the Department of Physics of the same University in 1992 working with L. Pietronero and A. Vespignani. He then moved to SISSA/ISAS in Trieste where he got the PhD in Statistical Physics in 1996 working on Self-Organized Criticality with A. Maritan. He has been postdoc in the Department of Physics in the University of Manchester with A. McKane and in TCM Group in the University of Cambridge with R. Ball. During his scientific activity in Rome he has also been visiting professor in the École Normale Supérieure in Paris, and in the Department of Physics of the University of Barcelona. After the studies on fractal growth and self-organized criticality he moved his research on the analysis of scale-free networks. On this topic he published a textbook and he coordinated a European Project (http://www.cosinproject.org).