Many states and utilities are targeting deep reductions in CO2 emissions from electricity generation. Recent and near-term reductions are being achieved by replacing coal generation with a mixture of solar, wind, nuclear, and gas generation, but there is little certainty about which technologies could be part of a least-cost portfolio to achieve low-to-no CO2 emissions. This research uses EPRI’s U.S. Economy, Greenhouse Gas, and Energy (US-REGEN) model to investigate how assumptions about technologies, markets, and policies can impact electric sector investments and costs for achieving low-to-no CO2 emissions. Modeling results suggest that abatement costs increase sharply for CO2 reductions beyond 80%, even assuming significant cost reductions for renewables and battery storage. The share of renewables generally increases with CO2 constraints, and wind and solar may comprise more than half of all generation even without policy support. However, the declining value of wind and solar at higher penetration means that least-cost decarbonization pathways include other technologies for many regions and scenarios. These include emerging technologies such as advanced nuclear, hydrogen, carbon-capture-equipped capacity, and long-duration energy storage, depending on the relative cost and availability assumptions. A “renewables only” decarbonization approach increases policy costs by about 50% relative to a full portfolio. Additionally, this analysis quantifies how the availability of carbon removal technologies can lower costs of meeting CO2 goals, especially for stringent targets.