In this talk we will review one of the most widely approaches -- generalized polynomial chaos (gPC). The gPC based methods employ orthogonal polynomials in random space and take advantage of the solution smoothness (whenever possible). The features of various gPC numerical schemes will be reviewed. Furthermore, we will discuss how real observational data can be utilized and combined with stochastic simulations. The resulting data-driven uncertainty analysis can provide much more insight to the true physics and produce predictions of high fidelity.