Increasing penetration of renewable resources in the grid pose several challenges to reliable operation. Utilities have begun deploying energy storage technologies as a means of mitigating some of these challenges. Although the presence of energy storage ameliorates the reliability challenges posed by intermittent sources, a quantitative assessment of the exact amount of storage required to meet a reliability target or guarantee in the presence of intermittent sources is not trivial. This presentation examines some of the challenges posed by renewable generation and means of quantifying their impacts on grid reliability, and discusses approaches to mitigate them using energy storage. Applications to three types of systems – standalone, island-capable, and interconnected – are presented. The presentation systematically develops a mathematical method for determining the amount of storage required to meet a reliability target at a specific load point, and extends it to a more complex island-capable microgrid system with solar generation, and a grid-connected system with wind generation. It discusses reliability metrics and targets, and a method for quantifying the notion of "firming" up an intermittent resource. Effects of resource availability and network constraints are considered. The presentation concludes with a discussion of another characteristic of renewable resources – low inertia – and how it impacts system reliability, and of ongoing research toward developing solutions for mitigating these impacts.