This book is the result of a growing realization that fluctuations play an active, even creative, role in nonequilibrium processes such as self-organization, pattern formation, and information processing and energy transduction in biological systems. In contrast to the general notion that fluctuations represent an undesirable "noise" in such systems - unimportant at best and undesirable or disruptive at worst - this book points toward a view in which fluctuations are fundamental aspects of the systems under study and frequently serve a constructive or stabilizing role in the dynamics. It includes contributions by prominent researchers in a great variety of disciplines, from theoretical cosmologists to experimental biologists, on such topics as stochastic resonance, noise in sensory nervous systems, mechanisms of intracellular transport, and kinetic theory of nonequilibrium systems.