This paper presents an analysis of the potential system-wide energy and air emissions implications of hydrogen fuel cell vehicle (H2-FCV) penetration into the U.S. light duty vehicle (LDV) fleet. The analysis uses the U.S. EPA MARKet ALlocation (MARKAL) technology database and model to simultaneously consider competition among alternative technologies and fuels, with a focus on the transportation and the electric sectors. Our modeled reference case suggests that economics alone would not yield H2-FCV penetration by 2030. A parametric sensitivity analysis shows that H2-FCV can become economically viable through reductions in H2-FCV costs, increases in the costs of competing vehicle technologies, and increases in oil prices. Alternative scenarios leading to H2-FCV penetration are shown to result in very different patterns of total system energy usage depending on the conditions driving H2-FCV penetration. Overall, the model suggests that total CO2emissions changes are complex, but that CO2emission levels tend to decrease slightly with H2-FCV penetration. While carbon capture and sequestration technologies with H2production and renewable technologies for H2production have the potential to achieve greater CO2reductions, these technologies are not economically competitive within our modeling time frame without additional drivers