Scientific medicine has a special pull on our imaginations. Like religion, it embraces our pain and our fears, and assures us that things can be better. And for all its missteps, it often fulfills its promise. You need only look back 20 years to see a world in which HIV/AIDS was essentially untreatable, depression went largely untreated and the U.S. death rate from heart disease was a third higher than it is today. Science has sparked transformations in each of those realms and now stands on the verge of even greater ones. As the stories in this NEWSWEEK Special Edition make clear, the prospects for improving human health have rarely been so bright. Yet even as we hurtle toward personalized prescriptions, stem-cell therapies and silver-bullet cancer drugs, the bedrock challenges of making medicine safe, affordable and accessible loom as large as ever.

What breakthroughs could the new century bring? For cancer patients, the excitement centers on a new generation of treatments designed not for massive conquest but for narrowly targeted strikes against tumor cells. Targeted therapy is an emerging ideal in psychiatry as well. Researchers are working to devise different treatments for different subtypes of depression--a trend that could help millions who get no relief from Prozac and its cousins--and applying the same principle to other afflictions as well. As science reveals more about the chemistry of mental function, diseases ranging from addiction to Alzheimer's could become as manageable as high blood pressure. With luck, several drugs that target the underlying mechanisms of Alzheimer's disease could reach the clinic before the first baby boomer turns 70.

That's just the beginning. The mapping of the human genome has set the stage for an era in which doctors use gene tests to determine which patients are most likely to benefit from a particular treatment or lifestyle regimen. And researchers are now working their way from the genome to the proteome--the vast array of biologically active protein molecules encoded by our DNA. Proteins are the microscopic workhorses behind everything from respiration to cogitation. By cataloging the 100,000 or so proteins that human genes produce, and pinpointing their functions, researchers will gain a surfeit of targets for drug molecules. And if the new art of therapeutic cloning fulfills its early promise, embryonic stem cells may someday help our ailing bodies produce whatever proteins they lack. The approach is still years from clinical use, but the tools are evolving fast. In an experiment reported this spring, South Korean researchers used DNA from ordinary skin cells to produce 11 lines of embryonic stem cells--each one genetically matched to its donor and theoretically capable of producing anything from insulin to dopamine.

The possibilities are endlessly seductive. But technological progress is not a complete recipe for better health, and there is real danger in equating newer with better. America has built the world's highest-tech medical system, yet the nation ranks 46th in life expectancy (behind Japan, Singapore, Canada and virtually all of Europe and Scandinavia). And 41 countries, including Cuba, have achieved lower rates of infant mortality. "Without systemwide health-care reform," says Dr. Henry E. Simmons of the non-partisan National Coalition on Health Care, "we're missing massive opportunities to create a healthier population."

New treatments can advance that cause, but they're only as good as our ability to manage them. Amid all the public debate over the ethics of stem-cell research, for example, there are safety issues to think about, too. Materials that originate in people or animals can spread everything from infections to malignancy, even when handled with some care. And as the British Medical Journal cautioned recently, stem-cell companies are now "springing up around the world with all the fervor of a new dotcom era." Costs are exploding, meanwhile, as technology expands and the population ages. Some 15 percent of the U.S. economy is now devoted to medical care, up from 10 percent in 1987. And America's uninsured population (45 million at last count) is growing in lock step with total expenditures. It doesn't take an expert to see where that trend leads. The Institute of Medicine estimates that 18,000 Americans now die every year for lack of health coverage.

What is a person to do? The forces shaping the health system are far beyond our reach as individuals, but those shaping our own well-being are not. Even as scientists explore the frontiers of biomedicine, they keep confirming the truism that health is easier to preserve than it is to repair. Wonder drugs aside, most of us can still achieve longer, better lives by exercising, eating well and managing our weight. In other words, medical science can light the path to optimal health. Walking it is still up to us.