Health care costs keep rising, and everyone—from government budget offices to insurance companies—wants to know how much we will spend on health thirty years from now. The problem is that the future of medicine is deeply uncertain: a single breakthrough drug or a new robotic surgery can completely rewrite the numbers. In this chapter we learn the main tools economists use to peer into that fog, why smooth trend lines often fail, and how to think clearly about the health technologies that do not yet exist.