Generation expansion planning aims to determine which generating  plants should be built to minimize the sum of the costs of investments   and operation over the planning horizon. Ensuring that this set of  generating plants meets adequacy criteria is a difficult problem  because a complex optimization must be combined with a detailed  simulation of the operation of the system. This presentation will  discuss how resource adequacy can be more closely integrated within  the optimization of generation capacity. As the proportion of variable  energy resources such as wind and solar increases, adequacy issues can  arise at any time of the year and no longer just during peak load  periods. We propose a technique to iteratively identify and integrate  critical risk periods in the optimization.