Challenges in complex systems science M. San Miguel, J.H. Johnson, J. Kertesz, K. Kaski, A. Díaz-Guilera, R.S. MacKay, V. Loreto, P. Érdi and D. Helbing Eur. Phys. J. Special Topics, 214 (2012) 245-271 Published online: 05 December 2012 DOI: 10.1140/epjst/e2012-01694-y