Overview: Unsolved problems of noise and fluctuations

Noise and fluctuations are at the seat of all physical phenomena. It is wel l known that, in linear systems, noise plays a destructive role. However, a n emerging paradigm for nonlinear systems is that noise can play a construc tive role-in some cases information transfer can be optimized at nonzero no ise levels. Another use of noise is that its measured characteristics can t ell us useful information about the system itself. Problems associated with fluctuations have been studied since 1826 and this Focus Issue brings toge ther a collection of articles that highlight some of the emerging hot unsol ved noise problems to point the way for future research. (C) 2001 American Institute of Physics.