Is the health of the U.S. population improving or getting worse, and how are health and medical costs influenced by obesity? How will anticipated advances in the biomedical sciences influence life expectancy and the cost of health care? The paper by Dana Goldman and colleagues is a daring speculation on the life-extending effects of possible future technologies-a valuable exercise given the speed of technological advances. Darius Lakdawalla and colleagues provide a methodologically solid basis for concluding that not only does obesity kill, it also takes an alarming toll on health and health care spending at levels that require immediate intervention. W h at w i ll t h e f u t u r e of U.S. health and longevity be? This is a question that scientists, pundits, and authors of science fiction novels have written about for centuries. Given the inevitable demographic wave of population aging now approaching the shoreline, estimating its magnitude and impact on the size of the beneficiary population and health care costs is of critical public policy importance. Papers in this Health Affairs online collection are focused squarely on this topic; here I discuss those by Dana Goldman and colleagues and by Darius Lakdawalla and colleagues.n Goldman and colleagues. The paper by Goldman and colleagues is an economist's view of how to model the future hazards of death and the financial costs of efforts to keep people alive using hypothetical technologies not yet in existence. 1 Think about this for a moment: This is not an easy exercise. First one has to come up with a list of technologies that do not yet exist but that might, and then one must estimate the impact these nonexistent technologies could have on the health and risk of death of today's younger generation when they are old enough to use them. To make such estimates, one has to pile one assumption upon another until what looks like a house of cards has been constructed. Is the result worth the risk of constructing such fragile models? In this case, I would have to say that it is.It is important to remember that this paper was not written as a projection model intended as a serious attempt to predict the future, but rather as a way to estimate the effect of hypothetical life-extending technologies on death rates and health care costs. If this were a genuine attempt to forecast the future, then the model and assumptions would have had to be extended to include other forces that could simultaneously increase the risk of death, such W 5 -R 8 6 2 6 S e p t e m b e r 2 0 0 5 P e r s p e c t i v e s