This paper examines both the potential of and barriers to grid-scale energy storage playing a substantive role in transitioning to an efficient, reliable and cost-effective power system with a high penetration of renewable energy sources. Grid-scale storage is a term that describes a number of different technologies with a wide range of characteristics. This versatility leads to the use of storage to perform a number of grid-services. We first enumerate these services, with an emphasize on those that are best suited to mitigate the effects of uncertainty and variability associated with intermittent, non-dispatchable renewable energy sources. We then provide an overview of the current methods to evaluate grid-integrated storage, summarize key findings, and highlight ongoing challenges to large-scale adoption of grid-scale energy storage. We focus on one particular area that is critical to both the efficient use of energy storage in the power grid and its long-term economic viability: the conflict between the technical benefits of this resource, which can provide both power and energy related grid-services (in some cases simultaneously), and the economic challenges of compensating these services within the current market structures. We then examine recent progress in addressing these issues through regulatory changes and other initiatives designed to mitigate previous market failures. This discussion is followed by some remarks about ongoing regulatory and market design challenges. The paper closes with a summary of the ideas presented and a discussion of critical research needs.