Uncertainty Modeling in Physical and Biological Systems With large-scale simulation reaching some degree of maturity we pose the more general question how to model uncertainty in physical and biological systems and how to propagate it. To this end, we propose a multi-element generalized Polynomial Chaos (ME-gPC) method that can overcome the difficulties associated with long-term integration, stochastic bifurcations and strong nonlinearities typically encountered in unsteady problems. Specifically, we extend the pioneering ideas of Norbert Wiener on polynomial chaos and our previous work on Wiener-Askey expansions in order to handle stochastic inputs with arbitrary probability distributions, either continuous or discrete. This fundamental development, in turn, allows us to introduce a subdomain decomposition of the random space, thus resolving adaptively large levels of localized random fluctuations and even stochastic discontinuities.