Beauty may be in the eye of the beholder, but over the years there have been some beauties that almost all beholders and critics have been able to agree on.
Grace Kelly in the 1950s, Cindy Crawford in the 1990s and these days it's Angelina Jolie or perhaps Kate Hudson, People magazine's "Most Beautiful" cover girl.
"Kate Hudson embodies beauty right now, she's adorable but she has flaws, her ears stick out a little . . . she's not a classic beauty like Michelle Pfeiffer," said Galina Espinoza, senior editor at People magazine.
It just so happens that Michelle Pfeiffer graced the cover of People's first Most Beautiful issue way back in 1990 when classic beauties and so-called "glamazons" such as Crawford were the hot "faces" of the day.
Fast-forward 18 years and women are gravitating to a more "accessible" kind of beauty, such as Hudson's features, according to Espinoza. And the list of accessible beauties is a long one -- think Reese Witherspoon, Drew Barrymore and Jennifer Anniston.
And if the standards of beauty have changed from classic to accessible in just 18 years, imagine the difference if you go all the way back to the 1700s.
"The most beautiful woman in London in 1720 who had a big mouth and a big nose would seem very ugly to us," said Lois Banner, a professor in the department of history at the University of Southern California.
Likewise, in the 1890s the Victorian ideal of beauty was really "a fat woman by today's standards, maybe about 5-foot-5 and 160 pounds, with big breasts and big hips," according to Banner.
Although fashion magazines had traditionally set the standard for what was beautiful, around the turn of the 20th century along came vaudeville performers and film stars to define the trends. Mary Pickford was one of the first silent-era film stars. Known as "America's Sweetheart" at the height of her popularity, Pickford's curly brown hair was widely copied by women around the country.
Then in the late 1920s along came two actresses whose looks defined an era, even though they could not have been more different.
"Joan Crawford was kind of a 'chick,' with her roguish eyes and rebellious ways," Banner said. Crawford's dark-haired beauty contrasted with Greta Garbo's angular features and thin physique.
"The word glamorous came in to use right about that time," Banner said, "and it was perfect to describe Greta Garbo."
But pretty soon both of those women took a back seat to the blonde bombshell, Jean Harlow. Patricia King Hanson, the film historian at the American Film Institute, said Harlow's looks were "very, very big. People wanted to have that platinum hair and the very pale skin and narrow eyebrow."
At the same time, the boyish attractiveness of Katharine Hepburn also was in vogue. "It was a straight up and down slim look. She was that way and so was Ginger Rogers," Hanson said.
World War II prompted a fashion change -- shorter skirts came into style because there just wasn't a lot of fabric to go around. War-weary GI's clamored for pretty girls and the "pinup" look was born. The more popular actresses were curvaceous women with big hips and big busts, a la Lana Turner and Betty Grable.
"They were gorgeous women, but they also had a distinctly American look. They looked much more American than European," Hanson said. The iconic photograph of the time showed Grable with her hands on her hips in a bathing suit looking over her shoulder.
And then along came Marilyn Monroe. Her platinum hair and sexy image got a lot of attention in the 1950s and early '60s. But Monroe had to share the role of image-maker with a host of classic beauties who were also making an impression, including Grace Kelly, Elizabeth Taylor and Audrey Hepburn.
"When I was in high school, we all wanted to look like Audrey Hepburn. When 'Charade' came out [in 1963] everyone wanted to own the little trench coat and look like her," Hanson said.
Although it seems like our concept of beauty changes radically from generation to generation, there are some consistent features, according to research done by Randy Thornhill, a biology professor at the University of New Mexico.
"People with symmetric faces are rated more attractive and always have been," Thornhill said.
In fact, research shows that when the left side of your face and the right side are well matched, people will find you attractive. Studies have also shown that babies prefer staring at the photos of symmetric faces than asymmetric ones. Along with symmetry, some evolutionary facial features are also key to the biology of beauty. Men prefer women with a small jaw, a small nose, large eyes and defined cheekbones. Among other things, females are looking for men with a strong jaw.
"The power of looks in social life has never changed from our evolutionary ancestors. If you look at cross cultures and traditional societies, you see the same sort of patterns. Attractive women get rewarded with more attractive husbands, husbands who are better providers," Thornhill said. Perhaps that explains Angelina Jolie and Brad Pitt.
These days, of course, many people try to "fool" Mother Nature with a little help from modern medicine.
"The only way you can change the symmetry of your face is through plastic surgery. You can trick the brains of others," Thornhill said.
Regardless of the powerful draw of symmetry, one way societies' concept of beauty has evolved is in its acceptance of different races and ethnic backgrounds.
At the beginning of the 20th century, beauty icons were almost exclusively Caucasian women. But by the 1980s, the Somalian-born model Iman captured the attention of fashion designers.
By the 1990s, a "most beautiful" list wouldn't be complete without Salma Hayek and Halle Berry.
The power of pop culture also has spawned a whole new genre of nontraditional beauties who vaulted to prominence by the sheer power of their fame.
Madonna, for instance, has facial features that could almost be considered unattractive -- a strong nose, for instance -- and yet millions of women have tried to emulate her looks.
And the American Film Institute's Hanson points out that actress Molly Ringwald starred in some of the most popular movies of the '80s, "Sixteen Candles" and "Pretty in Pink," sporting her trademark short red hair and freckles.
"She was not a great beauty, but she was very, very, popular in the 1980s and her looks were very attainable. You didn't have to think, 'I have to be look like somebody who is so perfect,' " Hanson said.
Not unlike Kate Hudson today.