In this chapter we present a broad overview of Operations Research and Management Science (OR/MS) methods and models used to support Environmental Decision Making (EDM) under uncertainty. We first survey the challenges and pitfalls frequently accompanying OR/MS applications to problems involving environmental issues. We consider and classify generic sources of uncertainty in quantitative models involving life support systems. We show how stochastic reasoning pervades some fundamental issues affecting decision making pertaining to the natural environment and environmental economics, in particular those related to discounting and intergenerational equity. We then discuss a selection of concepts and techniques that enable us to better understand and manage uncertainty in EDM. Finally, we indicate how the methods of stochastic control, stochastic programming, robust optimization and statistical emulation in meta-modeling can be used to shed light on some difficult issues arising in environmental decision making under uncertainty. This general discussion constitutes a preparation for the forthcoming chapters of this book.