Buttons never have been more important in fashion than they are for spring '89. And designers aren't just using them for practical purposes. The lowly button now is an important decorative touch that often dresses up an outfit as much as a piece of gorgeous jewelry. (If you want to look really "with it," buy a jacket or dress adorned with gleaming gold buttons. Instant chic!)

Diana Epstein, of course, has known about the button's fashion potential for a long, long time. But that isn't what attracts her. When she looks at one of the little fasteners, she sees culture, history, art - and one of the most fascinating hobbies in the world.Epstein and her partner Millicent Safro have enjoyed the gentle art of button collecting for nearly three decades and have made the hobby their livelihood. They run the only shop in the nation completely devoted to buttons.

A quaint and picturesque place, it's located in a diminutive brownstone in New York City. The building, circa 1854, once housed a miniature furniture store, and the name now above its door, "Tender Buttons," comes from the title of a book by Gertrude Stein. (Stein borrowed it from a sign she saw in a Paris market announcing the arrival of baby mushrooms.)

Epstein, a literary buff, came to New York from Chicago intending to follow in Gertrude Stein's footsteps. The button business happened by accident.

"I took a job in publishing and tried to write a novel in my spare time," she says. "For diversion, I started replacing the plastic fasteners on my clothes. One day I discovered a little shop run by an elderly Hungarian man that had the most marvelous selection of buttons. Well, I really don't know how it happened . . . but I ended up owning the place!"

Her friend Safro soon was in on the deal, and for the first few years Tender Buttons operated on 77th Street. Then they discovered the brownstone on 62nd, a place that seemed custom-designed for their business. They've been happily greeting their clientele there ever since.

The clientele comes from all over the world, all walks of life. Epstein notes that's one of the most fascinating things about the profession - the people who come through the door eager to browse and buy.

Fashion designers, Broadway and Hollywood stars, socialites, antique collectors, individuals seeking the different and the off-beat - Tender Buttons gets them all. And, oh, the unusual requests!

Recalls Epstein with a smile: "We've had people in here looking for buttons made in the shape of frogs; people who won't settle for anything but rare jeweled buttons; people who want incredibly odd sizes. Once we had a woman who was determined to find just the right buttons for her dog's trench coat!"

No matter how strange or difficult the requests happen to be, the business partners try to fill them. So, they're constantly traveling and searching for interesting buttons to beef up their stock. Some are discovered in attics, at antique sales or estate auctions. Others catch their eyes at collectors' conventions. The two also have managed to find a few craftsmen who still can turn out buttons that look more like art pieces than fasteners, and they frequent their studios and factories.

"Millicent designs a great many of the buttons that we sell, and we have them produced by these craftsmen," Epstein says. "Millicent also designs jewelry that incorporates buttons, and we feature it here at the store."

At one time, it wasn't nearly as difficult to find beautiful and unique buttons. But as the years have gone by, plastic has taken over and natural materials - quality materials - have become more and more scarce.

"We're in business to promote quality, preserve tradition and to help people rediscover a more genteel time," Epstein explains.

Step into the shop, and you sense tradition on every side. Indeed, in many ways, Tender Buttons seems more like a museum or art gallery than a store. Numerous examples of rare and interesting buttons are framed and mounted on the walls. Window displays, changed with the seasons and holidays, are delightfully creative. And pricing's equally clever. One drawer full of inexpensive buttons - buttons shaped like chicks - is marked "cheep."

Just how many buttons are in the inventory?

Epstein has no idea. Millions, she believes - and in a variety of materials including pearl, horn, wood, coconut and bamboo.

They even have some plastic buttons that are quite beautiful.

Upstairs are even more beautiful buttons. And this is where Epstein spends much of her time - cataloging the treasures that fill countless boxes and shelves (there are 18th century jeweled buttons, buttons by famous paperweight makers, garter buttons, aluminum buttons, ceramic buttons, ivory buttons, enchanting French buttons with hand-painted scenes, buttons featuring signs of the Japanese zodiac etc., etc., etc.)

There probably isn't a subject in the world that hasn't been put on a button sometime in history, she notes. From fairy tales, to war, to presidential inaugurations - everything's there.

The old buttons "speak" to her. She simply can't pass by without touching, without admiring, without paying proper respect. And she never tires of researching their past. Perhaps her most interesting discovery: Stone fasteners were used on hides and furs way back in prehistoric times. The button's roots are ancient.

"I find everything about buttons intriguing," Epstein says. "I think many other people do, too, because we're constantly attracting more customers. But I'll tell you something. I don't want this business to get too big or too commercial. The world's already such a big, complex place . . . we need small and charming things to help us keep our equilibrium."