Building transmission to reach renewable energy (RE) goals requires coordination among renewable developers, utilities and transmission owners, resource and transmission planners, state and federal regulators, and environmental organizations. The Western Renewable Energy Zone (WREZ) initiative brings together a diverse set of voices to develop data, tools, and a unique forum for coordinating transmission expansion in the Western Interconnection. In this report we use a new tool developed in the WREZ initiative to evaluate possible renewable resource selection and transmission expansion decisions. We evaluate these decisions under a number of alternative future scenarios centered on meeting 33% of the annual load in the Western Interconnection with new renewable resources located within WREZ-identified resource hubs. Of the renewable resources in WREZ resource hubs, and under the assumptions described in this report, our analysis finds that wind energy is the largest source of renewable energy procured to meet the 33% RE target across nearly all scenarios analyzed (38-65%). Solar energy is almost always the second largest source (14-41%). Solar exceeds wind by a small margin only when solar thermal energy is assumed to experience cost reductions relative to all other technologies. Biomass, geothermal, and hydropower are found to represent a smaller portion of the selected resources, largely due to the limited resource quantity of these resources identified within the WREZ-identified hubs (16-23% combined). We find several load zones where wind energy is the least cost resource under a wide range of sensitivity scenarios. Load zones in the Southwest, on the other hand, are found to switch between wind and solar, and therefore to vary transmission expansion decisions, depending on uncertainties and policies that affect the relative economics of each renewable option. Uncertainties and policies that impact bus-bar costs are the most important to evaluate carefully, but factors that impact transmission costs and the relative market value of each renewable option can also be important. Under scenarios in which each load zone must meet 33% of its load with delivered renewable energy from the WREZ-identified resource hubs, the total transmission investment required to meet the 33% west-wide RE target is estimated at between $22 billion and $34 billion. Although a few of the new transmission lines are very long—over 800 miles—most are relatively short, with average transmission distances ranging from 230- 315 miles, depending on the scenario. Needed transmission expenditure are found to decline to $17 billion if wide use of renewable energy credits is allowed; consideration of renewable resources outside of WREZ-identified hubs would further reduce this transmission cost estimate. Even with total transmission expenditures of $17-34 billion, however, these costs still represent just 10-19% of the total delivered cost of renewable energy.