As illustrated in the previous chapters, the study of fluctuations that began with Einstein’s and Smoluchowski’s theory of Brownian motion has been fundamental for the development of statistical mechanics and, more recently, for its applications to a range of fields that extends well beyond physics. In the first place, Einstein’s relation, that connects diffusivity and mobility Einstein (1956), was immediately found to apply to a much wider class of phenomena, than specks of dust or pollen suspended in a fluid, to include, e.g., fluctuations of electrons in an electrical circuit.