This is a volume in honor of Professor Peter Carruthers on the occasion of his 61st birthday. It is a unique collection of papers by the world's leading experts, describing the most exciting developments in many areas of theoretical physics. While traditionally physics is driven to ever smaller and simpler systems, end-of-this-century scientists see themselves confronted with complex systems in many of their areas. It is just this interdisciplinary character of complexity that is addressed in this book, with topics ranging from the origin of intelligent life and of universal scaling laws in biology via heartbeats, proteins, fireballs, phase transitions, all the way to parton branching in collisions of elementary particles at high energies. The contributions include extensive discussions on complexity (M Gell-Mann, M Feigenbaum, D Champbell, D Pines and L M Simmons), neutrino masses (R Slansky and P Rosen), high temperature superconductors (D Pines), low Moon (M Feigenbaum), origin of intelligent life (S Colgate), chaos of the heart (M Duong-Van), origin of universal scaling laws in biological systems (G West), critical behavior of quarks (R Hwa), status of LEGO (S Meshov), disoriented chiral condensate (F Cooper), and many others.