Almost all activities in real life entail different kinds of uncertainty. From daily decisions to complicated problems, such as petroleum reservoir characterization, suffer from uncertainties. Uncertainty can have different roots, including incomplete observation of the system, incomplete modeling of the system because of our limited knowledge and understanding of the underlying mechanisms and rules of the system, intrinsic uncertainty in the system, and inaccurate measurement of the system’s parameters. The first step to deal with uncertainty is to recognize its root and type. This chapter introduces different types of uncertainty in reservoir engineering, challenges induced by uncertainty, and common ways to treat them. Two general approaches to handle the uncertainty are described, named forward uncertainty management and inverse uncertainty management. Forward uncertainty management tries to propagate the uncertainty from inputs to the output(s) to make robust decisions regarding the problem understudy. On the other hand, inverse uncertainty management deals with calibrating model parameters to reduce the range of uncertainty.