The ultimate performance of ITER has the potential to exceed the nominal levels needed to meet the objectives of the Physics and Technology phases, as outlined in the ITER Terms of Reference. Higher power levels, even with the existing set of physics design rules, may be achievable with modifications to torus components and appropriate additions to the balance of plant. It may also be possible to generate net electric power from a machine the same size as the current ITER baseline, but with a slightly different design. Because of the large investment in ITER and the value of the information gained from its operation to the progress of fusion research, it is important that the operation and performance of the machine be maximized. The greater value of information that could be obtained with more ambitious performance levels must be weighed against the additional costs, technological risks, and safety implications. This study examines the feasibility and implications of a potential third phase, or Advanced Technology Phase (ATP) for ITER. Performance prospects for this phase, under certain assumptions, have been assessed. Impacts on other systems, other components, safety, and configuration have been assessed. The study shows that net electric power can be obtained, but innovative divertor designs are needed, along with changes in the heat transport system, shielding, and machine configuration. The net electric power produced comes with the risk of increased safety concerns, and additional costs. Net power generation from a single sector (1/16) of the machine is also considered. In terms of cost, complexity, and risk, this may be a more desirable option for demonstrating net electric power production.