The long economic lifetime and development lead-time of many electric infrastructure investments requires that utility resource planning consider potential costs and risks over a lengthy time horizon. One long-term -- and potentially far-reaching -- risk currently facing the electricity industry is the uncertain cost of future carbon dioxide (CO2) regulations. Recognizing the importance of this issue, many utilities (sometimes spurred by state regulatory requirements) are beginning to actively assess carbon regulatory risk within their resource planning processes, and to evaluate options for mitigating that risk. However, given the relatively recent emergence of this issue and the rapidly changing political landscape, methods and assumptions used to analyze carbon regulatory risk, and the impact of this analysis on the selection of a preferred resource portfolio, vary considerably across utilities. In this study, we examine the treatment of carbon regulatory risk in utility resource planning, through a comparison of the most-recent resource plans filed by fifteen investor-owned and publicly-owned utilities in the Western U.S. Together, these utilities account for approximately 60percent of retail electricity sales in the West, and cover nine of eleven Western states. This report has two related elements. First, we compare and assess utilities' approaches to addressing key analytical issues that arise when considering the risk of future carbon regulations. Second, we summarize the composition and carbon intensity of the preferred resource portfolios selected by these fifteen utilities and compare them to potential CO2 emission benchmark levels.