Clean energy technologies that cost more than fossil fuel technologies require support through research and development (R & D). Learning-by-doing relates historical cost decreases to accumulation of experience. A learning investment is the amount of subsidy that is required to reach cost parity between a new technology and a conventional technology. We use learning investments to compare the relative impacts of two stylized types of R & D. We define curve-following R & D to be R & D that lowers costs by producing knowledge that would have otherwise been gained through learning-by-doing. We define curve-shifting R & D to be R & D that lowers costs by producing innovations that would not have occurred through learning-by-doing. We show that if an equal investment in curve-following or curve-shifting R & D would produce the same reduction in cost, the curve shifting R & D would be more effective at reducing the learning investment needed to make the technology competitive. The relative benefit of curve-shifting over curve-following R & D is greater with a high starting cost and low learning rate. Our analysis suggests that, other things equal, investments in curve-shifting R & D have large benefits relative to curve-following R & D. In setting research policy, governments should consider the greater benefits of cost reductions brought about by transformational rather than incremental change.