This book surveys recent advances related to the application of single molecule techniques in various fields of science. The topics, each described by leading experts in the field, range from single molecule experiments in quantum optics and solid-state physics to analogous investigations in physical chemistry and biophysics. A unifying theme of all chapters is the power of single molecule techniques to unravel fluctuations and heterogeneities usually hidden in the ensemble average of complex systems. The concept for the book originated from a gathering of some of the world's leading scientists at the Nobel Conference in Sweden.