(FORTUNE Magazine) – The alarm finally goes off in your head around 3 p.m. Your face flushes and your hands plow through the papers on your desk. You have accidentally stood someone up for lunch. It gets worse. You can't remember who. And still worse: You can't recall where you left your glasses, so you can't look up the name in your appointment book. This is the afternoon you find yourself at a different place in life. Ten years ago, when you were 40, you would not have--could not have--forgotten anything.

Why do our memories betray us? Is this a precursor of Alzheimer's or some other serious mental disorder? How can some people command a loyal and prodigious memory well into old age? Are there ways to make everyone's memory clear again?

First, reassurance: A momentary loss of memory is most probably not a sign of Alzheimer's, or if so it's a very distant one. People between 65 and 75 face only a 4% chance of suffering from that sad, destructive disease, vs. a frightening 50% chance for those over 85 (see Alzheimer's box). Yet almost all of us will be tripped up by forgetfulness from time to time as we age. Memory may begin to get a little shaky even in our late 30s, but the decline is so gradual that we don't start to stumble until we're 50ish.

The vanguard of 78 million baby-boomers will be 49 this year, so an ever larger share of the population will be turning desktops upside down. For many, their anxiety in already difficult careers could rise significantly. Moderate memory loss may be easily manageable for those who spend their entire working lives in the same company. In that steady-state universe, new people arrive and rules change slowly. The 45-year-olds who are downsized out and working as consultants, on the other hand, suddenly must master the rosters of half a dozen clients and as many ways of doing business.

Neuroscience, in a timely way, has begun to pay more attention to this condition. Researchers call it AAMI, age-associated memory impairment. The Charles A. Dana Foundation, named for an early manufacturer of differential joints for cars, has given $8.4 million to five major university medical centers to study AAMI. Researchers get the help of powerful instruments like PET (positron emission tomography) scanners that can detect the chemical changes taking place in the brains of subjects as they perform such tasks as memorizing vocabulary lists.

Some 450 middle-aged and elderly volunteers visit Johns Hopkins Hospital in Baltimore once a year to take a series of memory exams, the results of which are tracked over decades. The brains of some of these good sports will be examined after they die to see if their declining scores over the years relate to physical signs of disease and atrophy.

Much about memory is still baffling. "Despite all the noise we scientists make about memory, it is remarkable how little we know," says Dr. Arnold Scheibel, director of the UCLA Brain Research Institute. He and his colleagues can be forgiven. The brain has as many as 100 billion neurons, many with 100,000 or more connections through which they can send signals to neighboring neurons. The number of potential pathways would be beyond the ability of the most advanced supercomputers to map.

Some of the predetermined roadways seem bizarre. In early February, for example, researchers discovered that men process language in one part of their brain, women in several. As for memory, the names of natural things, such as plants and animals, are apparently stored in one part of the brain; the names of chairs, machines, and other man-made stuff in another. Nouns seem to be separated from verbs. (That may explain the resistance of some brains to neologisms that turn nouns into verbs, such as "Let's dialogue on this" or "I'll liaise with Helen's team.")

Aspects of memory are scattered throughout the brain, but many researchers believe the hippocampus (Greek for "sea horse," the shape of the tiny organ) has an especially important role. That is where new information is turned into memory. How memories are made--and fade--is still mysterious. But this much is known. Neuron No. 28, say, fires an electrical signal, and in the synapse where one of 28's connectors touches a receiver of neuron No. 29, a chemical change takes place that triggers an electrical signal in 29. That signal gets passed on to neuron No. 30, and on and on. If the connection between 28 and 29 is made often enough, the bond between the two neurons grows stronger. This crucial marriage, the stuff that memory seems to be made of, neuroscientists have dubbed, unpoetically, long-term potentiation , or LTP.

Though memories may be created in the hippocampus, they are stored elsewhere. In TV soap operas the amnesiac is the ingenue who has forgotten she is already married to her fianca's brother in another city, but is otherwise able to function more or less normally. That doesn't happen often, if ever. (For other memory myths, see box.) Real amnesiacs are people who remember the past but not the present. Their hippocampi have been severely damaged, so they are unable to form new memories, but most old memories remain intact.

Daniel L. Schacter, 42, a Harvard psychology professor, played a round of golf with one such victim. M.T., who was 58, remembered the rules and all the lingo from bogie to wedge. But he couldn't recall where he hit his ball. If Schacter drove first and M.T. followed, M.T. had half a chance of holding on to the image of where his ball went long enough to track it down. But if M.T. drove first and had to wait for Schacter to drive, he had no chance. After M.T. walked off one green, Schacter noted in his journal, "the patient was surprised and confused when told he had not yet putted."

Amnesia can be caused by a virus, a blow to the head, a near drowning or stroke that deprives the brain of oxygen for a time, or a faulty gene that programs parts of the brain to deteriorate early. Stress can play a part as well. Lab animals exposed to low levels of shock they cannot control produce glucosteroids that damage their hippocampi. Marilyn Albert, 51, a researcher at Massachusetts General Hospital, notes that among the elderly she is studying in the Boston area, those who are less educated, are less active physically, and feel less able to influence what happens to them day to day tend to experience greater memory loss than the better educated who regard themselves as more commanding. (This kind of stress is not the same as pressure to finish a job or perform well. Pressure can stir strong emotions that actually help imprint memories more deeply.)

As we age, most of us will experience at least some slowdown in ability to remember. What do we have to fear and how do we avoid it? Laypeople are accustomed to distinguishing between long-term and short-term memory. That oversimplifies the phenomenon. Dr. Murray Grossman, 43, a University of Pennsylvania Medical Center neurologist, has helped develop a model that separates memory into five types (see illustration). He assigns each a locale, or a possible locale, in the brain and assesses the likelihood of each type's decaying over time. In order of durability, the memory types are as follows:

SEMANTIC. The memory of what words and symbols mean is highly resilient--even some Alzheimer's patients retain much of their semantic memory. It's unlikely you'll forget what "Tinkertoy," "prom," and "mess hall" mean even though you haven't used the words in years. Nor do you forget religious symbols and corporate trademarks or what distinguishes a cat from a dog. You can add words to your semantic memory until death.

IMPLICIT. Years ago someone taught you to ride a bike. You may not recall the specific instructions of those wobbly, knee-banging first outings, but you will not forget all that you have learned about bike riding over a lifetime--without even being conscious of learning--from turning corners at high speeds to stopping on a dime. How to swim or drive a car and many other skills that depend on automatic recall of a series of motions don't disappear either. Nor do conditioned responses. Like Pavlov's dogs, once you've learned to salivate at the sound of a bell, you'll do it forever. Nor will you neglect to reach for a handkerchief when you sense a sneeze, or for a dollar bill when you see a doorman. Loss of implicit memory is a sure sign of serious mental deterioration.

REMOTE. This is the kind of memory that wins money on Jeopardy. It is data collected over the years from schools, magazines, movies, conversations, wherever. Remote memory appears to diminish with age in normal people, though the decline could be simply a retrieval problem. "It could be interference," says Johns Hopkins neurologist Dr. Barry Gordon, 44. "We have to keep sorting through the constant accumulation of information as we age."

When a 60-year-old hears "war," it has many more associations than Vietnam or the Gulf. And compared with the 30-year-old, the 60-year-old may have to rummage through twice as much data before digging way back to the lessons of a high school history course and finding the names of the five Presidents after Lincoln.

WORKING. Now we enter territory that does erode, at least for most people. This is extremely short-term memory, lasting for no more than a few seconds. It is the brain's boss, telling it what to cling to. In conversation, working memory enables you to hang on to the first part of your companion's sentence while she gets to the end. It also lets you keep several things in mind simultaneously--to riffle through your mail, talk on the phone, and catch the attention of a colleague walking by the door to ask him if he wants to go to lunch--all without losing your place.

For reasons that aren't altogether clear, working memory in many people starts to slow down noticeably between 40 and 50. "Certain environments become more difficult, like the trading floor of a stock exchange, where you have to react very fast to a lot of information," says Richard Mohs, 45, a psychiatry professor at Mount Sinai School of Medicine in New York City. Jetfighter combat is out.

EPISODIC. This is the memory of recent experience--everything from the movie you saw last week and the name of the client with whom you booked lunch to where you put your glasses--and it too dwindles over time. This is the form of memory loss, the AAMI, that does, or will, trouble most people. You remember how to drive your car, but that's academic because you can't recall where you parked it.

Episodic memory could begin to dwindle in the late 30s, but the downward glide is so gentle that unless you are trying to memorize the Iliad or pass a bar exam, you probably won't notice for a couple of decades. At 50, however, you are likely to feel a little anxiety as you watch the younger people in the office, even the non-techies, learn how to operate the new computer software much more quickly than you do.

Several years ago a Massachusetts insurance company, observing that malpractice suits are brought against old doctors more often than young ones, asked researchers to develop tests for identifying physicians at risk. Dean K. Whitla, 69, a Harvard psychologist, was on a team that examined 1,000 doctors, ages 30 to 80. In one test the subjects were seated in front of computers and asked to read stories crammed with details, such as street addresses. A few minutes later they took a multiple-choice test.

Ability declined steadily with age, says Whitla. Though some of the 80-year-olds were as good as the 30s, on average the 80s could remember only half as much as the 30s. But there were also some 80s who on further investigation couldn't match the patients they had seen that day with their complaints. (The insurance company has not yet disclosed whether it plans to act on the results.)

What's going on up there? Unlike cells elsewhere in the body, neurons don't divide. They age, and at the rate of 100,000 a day they die, says Dr. Daniel Alkon, 52, chief of the neural science lab at the National Institutes of Health. By the time someone reaches 65 or 70, he may have lost 20% of his 100 billion. Return to the hippocampus, where episodic memory is first recorded. Neuron 28 and some of its neighbors may be dead or so feeble they no longer transmit electrical charges efficiently.

Still, 80 billion remaining neurons is a lot. And even though the brain cannot grow new ones, the neurons can likely sprout new synapses late into life and thereby form new connections with one another. William Greenough, 50, a researcher at the University of Illinois, supplied lab rats with new balls, dolls, and other toys to play with daily and changed the chutes and tunnels in their cages. When he cut open their brains, he counted many more synapses than in rats that got no toys and no new decor.

It's a good guess that the human brain, too, grows more synapses when stimulated and challenged. So the brain-even while shrinking-may be able to blaze ever more trails for laying down memory. If the neuron 28 path is no longer easily passable, the number of alternate routes may be virtually limitless. The trick is to force the brain to make them.

The habits of highly intelligent people offer a clue as to how to do that. By and large, says Harvard's Schacter, the higher people score on the Wechsler Adult Intelligence Scale (100 is the mean), the higher they score on the Wechsler Memory Scale. "Memory depends on processing," he says. "Very smart people process information very deeply." Perhaps they relate a magazine article on memory to a book on artificial intelligence and a play about prison camp survivors. Doing so, they could be laying networks of neuron highways that will make the recollection of the article, book, or play accessible by multiple routes.

With effort, people with average intellects can boost their memories substantially. For example, most people have trouble remembering numbers of more than seven digits or so, a limitation long recognized by telephone companies. But a decade ago, researchers at Carnegie Mellon University trained otherwise undistinguished undergraduates to memorize hundred-digit numbers. Focusing hard on that long string of digits, the students found patterns they could relate to meaningful number series, such as birthdays.

Forgetting names bedevils most people, the more so as they age. So meet Harry Lorayne, 68, a memory coach and theatrical wonder already familiar to many insomniacs. His half-hour TV infomercials with Dick Cavett run at 4 a.m. and other off-price times. Lorayne has also appeared on the Tonight show and memorized the names of as many as 500 people in the audience. His gift is that he can quickly invent a dramatic, often grotesque, image to slap on the face of everyone he encounters. "I meet Mr. Benavena, and I notice he has a big nose," rasps Lorayne in a voice that was trained on New York's Lower East Side. "So I think 'vane,' like weather vane, a nose that's a bent weather vane." Lorayne's Memory Power package of videotapes, audiotapes, and a book sells for $115.

Frank Felberbaum, 58, refers to himself as a corporate memory consultant. "Think of bottles of beer falling like bombs," he introduces himself unforgettably. Felberbaum's clients include GE Capital, Conda Nast, and some Marriott hotels. For about $6,000, Felberbaum trains a group of 20 or so executives in a two-day course that instructs them on how to retain such critical data as a range of interest rates and the names of hotel guests.

The methods of Lorayne and Felberbaum are legitimate, say the neuroscientists. The routines they teach--fastening names and other information to vivid pictures--have been around since the ancient Greeks. Lorayne likes to trace his intellectual roots to Aristotle, who taught that in order to think, we must speculate with images. Matteo Ricci, a 16th-century Jesuit missionary to China, "built" a memory palace in his mind and wandered the halls, storing the dosage for a new medicine in one room, perhaps, and retrieving a Thomistic proof for the existence of God from another.

There are modest ways to build, if not a palace, at least a comfortable home for memory. College students may be superior at memorizing not only because their neurons are young but also because they develop mnemonic devices to survive exams. That's an easy practice to resume. For example, memory is WIRES--working, implicit, remote, episodic, and semantic. One of the clichad pieces of advice for improving your brain, including memory, is to marry someone smarter than yourself. If that's inconvenient, at least hang out with challenging, fast-thinking company. Or study accounting, zoology, or a new language.

Coming someday, perhaps, is a memory pill. Cortex Pharmaceuticals, founded by three neuroscientists from the University of California at Irvine, claims to have developed a class of drugs called ampakines that revive tired neurons. Gary Lynch, 52, one of the founders and a prominent LTP researcher, says ampakines heighten the ability of the remaining receptors in weakened neurons to carry on after some of their synapses have died. "We know that this works in middle-aged and old rats," says Lynch. "If you give them ampakines, they will remember in the afternoon where they found food in the morning."

Cortex President Alan Steigrod, 57, says that preliminary clinical trials on humans in Germany have been encouraging. The company hopes to test the drugs soon on about 100 Alzheimer's victims in the U.S. Ampakines, or another series of drugs, may eventually prove to be the easy, safe, and effective way to freshen old memories. Or they may not. A Salk Institute researcher questions whether they are any more useful than caffeine. And they might have dangerous side effects. So the Food and Drug Administration could approve ampakines for Alzheimer's sufferers, who don't have much to lose, but keep them off the market for a long time for those afflicted by normal memory loss. Waiting for the FDA's okay, you could probably learn Chinese.