Renewable electric power sources like wind and solar have been shown from a resource perspective to have significant potential to reduce the carbon dioxide emissions associated with the electric power sector. However, the intermittency of these resources is often cited as a barrier to their large scale integration into the grid. In this review, we provide a framework for understanding the body of literature that has been devoted to the behavior and reliability of intermittent renewables and discuss recent grid integration analyses within this framework. The modeling approaches required for system characterization are found to depend on the energy penetration of the intermittent technology and recent simulations reveal substantially different behavior in low- and high-penetration regimes. We describe an analytical approach that addresses both penetration regimes and can be used to incorporate the results of grid integration studies into decarbonization strategy analyses.