These are some of the questions that I find come up over and over again when I talk about olive oil with Americans:

So, what exactly is extra-virgin olive oil? Does it grow on special trees? Is it made from special olives?

No, extra-virgin olive oil can be made from almost any of the thousands of varieties of olive that grow around the Mediterranean basin. What distinguishes extra-virgin olive oil from ordinary olive oil is not how it's grown but the way it is produced-from sound, healthy fruit, harvested when it is somewhere between half-ripe and fully ripe. Within hours of harvest, the olives must be pressed, and for extra-virgin oil they can only be pressed mechanically, using no chemical solvents to extract the oil. The freshly pressed oil is then separated from the vegetable water that occurs naturally in the olive fruits. Finally, only if the free oleic fatty acid content of the oil is below the minimum (0.8 %) set by the International Olive Oil Council, and if the oil is judged by a tasting panel to be free of defects, only then it can be called extra-virgin.

Well, what is regular olive oil then?

Regular olive oil (which, confusingly, used to be called pure olive oil) is oil that is disqualified from extra-virgin because of unpleasant aromas or flavors or other objectionable qualities, such as a free oleic fatty acid content above 0,8%. The oil can still be used but it must be treated with solvents to remove aromas and flavors, then a small amount of extra-virgin or virgin oil may be added back to give the oil some character.

So isn't one just as good as the other?

No. Extra-virgin olive oil is always to be preferred. Both oils are high in monounsaturated fatty acids, but that is just one part of what makes olive oil good for us. Extra-virgin oil, because it is untreated, is rich in valuable polyphenols, indicators of antioxidants that are a major reason for the importance of olive oil in maintaining good health. Extra-virgin olive oil is the foundation of the healthy Mediterranean diet; plain olive oil is a modern invention.

How does that work? What role does extra-virgin olive oil play in a healthy diet?

First of all, monounsaturated fats work in the human body to reduce harmful LDL cholesterol and stabilize or even increase beneficial HDL cholesterol. But beyond that, the antioxidants in extra-virgin oil (but not in plain olive oil) act as scavengers in the blood, scooping up the free radicals that we all produce just by the very act of being alive, but that increase with activities like smoking or breathing polluted air. So the monounsaturated fats help to protect against heart disease, while the antioxidants shield us against many types of cancer.

What is the best olive oil for me to buy?

That's like asking what's the best cheese? It all depends on your own personal taste and what you want to do with the oil. Many people prefer the sweet, rather simple flavors of late-harvested oil from the Ligurian region of Italy, for instance, while others find them overly bland; those people are more intrigued perhaps by the robust flavors of oils from Central Italy, Tuscany and Umbria, or the fruity flavors of Southern Italian oils, like those from Puglia, Sicily and Calabria-and Ligurians might well find those oils to be unacceptably aggressive. The only way to tell is by tasting, tasting, tasting.

Moreover, you may well find that an oil that is terrific with a poached fish (like those Ligurian oils) is not sufficiently full-bodied to stand up to a mixed grill of onions and sweet red peppers. Or an oil that is perfect for making a tomato sauce is a little overwhelming when used on a salad. In fact, in Italy many cooks keep two oils, one for all-purpose cooking and another, perhaps more delicate one, for garnishing. The only way to tell is through experience.

Did you say "cooking?" I've heard you can't cook with extra-virgin olive oil.

This is probably the most egregious misunderstanding of olive oil that exists in America, and unfortunately it is pervasive in both professional and domestic kitchens. Somehow, the myth has been broadcast that extra-virgin oil has a low smoke point, or flash point, and is therefore unacceptable for cooking. Nothing could be further from the truth, as thousands of cooksall over the Mediterranean, including many of the region's finest chefs, prove every day of the year when they fire up their cooking pots and add extra-virgin olive oil to them. Even deep-fat frying is no problem with extra-virgin oil-the best temperature for deep-fat frying is around 180ºC (365ºF.)-extra-virgin olive oil presents no problems whatsover at that temperature. Moreover, many cooks feel there's an additional virtue to using extra-virgin oil for frying in that it gives a crisp, crunchy texture that is not produced by other oils.

Obviously you would not use a fine estate-bottled extra-virgin for deep-fat frying, any more than you would use a Brunello di Montalcino to make a beef stew. But just as there are plenty of excellent vini da tavola that are appropriate for cooking, so there are many, many extra-virgin oils that are cheaper and better for frying.

But you must have a few buying tips-especially for people who know nothing about olive oil?

One of the most important things to look for in purchasing olive oil is a harvest date or a "use by" date. Unlike that fine Brunello, olive oil doesn't improve with age. Fortunately, more and more conscientious producers date their oils. Try to avoid oils that are more than two years past their harvest date-they will not be bad oils but they will no longer be in their prime. Unfortunately, a number of sins are committed at the retail end of olive oil sales by shop owners who are ignorant of how olive oil should be handled and insist on displaying clear or almost clear glass bottles of oil in sunny shop windows. Never, ever buy that oil-don't even accept it as a gift since it is just about worthless. The ideal situation is a shop that has a display of single bottles of each oil being sold, with the actual products kept in a cool, dark back room. Alas, very few retail shop keepers are in a position to do that, or to understand the importance of doing that.

Beyond that, take advantage whenever you see an opportunity to taste olive oil. Smart retailers will very often have a sample available for customers. If you're not sure of an oil, buy several bottles in small quantities and get to know them before moving on. Above all, keep reading and studying-there's loads of useful information on the Internet and the food press is always discovering some new oil that promises to deliver ecstacy, or something close to it.

And use the olive oil you buy-use it to taste, to cook with, to offer to friends. There are many, many ways to add olive oil to a healthy diet beyond just dressing a salad. Nothing beats a freshly baked potato with a good dollop of extra-virgin olive oil instead of butter on top, and even that great American treat, a steamed or roasted ear of sweet corn is a revelation when rolled in extra-virgin olive oil, perhaps with some freshly minced garlic, salt and pepper mixed in.