In Pictures: The 20 Richest Women In Entertainment

Of course, the celebrities with the longest careers proved the most daunting to evaluate. Since beginning her career in the early 1980s, Madonna has sold some 200 million albums worldwide and starred in a string of largely disappointing films. She owns an impressive portfolio of properties and briefly ran her own record label, Maverick, a part of Warner Music Group. Our estimate of her net worth--$325 million--is definitely on the conservative side.

Indeed, most of the women on our list cull their earnings from multimedia enterprises. Jennifer Lopez, for example, boasts valuable perfume and fashion lines in addition to her film and music careers; Harry Potter scribe J.K. Rowling now enjoys millions of dollars in royalties and merchandising revenues from the incredibly successful film adaptations of her books; and supermodel Gisele Bundchen, a fixture of Victoria's Secret catalogs, earned her $70 million fortune not just from modeling, but also from a line of successful sandals sold in Brazil.

The youngest women on the list? Twenty-year-old twins and former child stars Mary-Kate and Ashley Olsen, who have converted their supporting roles on a middling 1980s sitcom into a multimillion dollar retail empire called Dualstar Entertainment. The twins are often credited in the media as presiding over a fortune of as much as $1 billion. Not so. Despite Dualstar's retail sales of $1 billion, the waifish mini-moguls don't pocket all of it. We estimate their combined net worth at around $100 million.

Britney Spears, 25, also makes the list, despite a relatively brief career that took off only in 1999 with the release of her hit single "...Baby One More Time." Her $100 million fortune was earned largely from music--she has sold over 75 million albums to date--but she has supplemented her income with a profitable line of perfumes and multimillion-dollar endorsement deals.

The richest actress on the list is Julia Roberts, who built her estimated $140 million fortune film by film. The Pretty Woman star was the first actress in Hollywood to command a $20 million-per-film paycheck, a fairly common salary for male superstars like Will Smith and Johnny Depp. Other actresses who make the list largely due to their film fees are Nicole Kidman and Cameron Diaz. Sandra Bullock, who checks in at No. 14 on the list, supplemented movie earnings by producing the ABC sitcom The George Lopez Show, which went into syndication last year.

Martha Stewart is the only woman on the list whose net worth fluctuates just about every second. Her nearly $650 million fortune is based almost entirely on the 28 million shares she owns in Martha Stewart Living Omnimedia (nyse: MSO - news - people ). (In 2005, that stake was worth $1 billion.) Though her brief stint in prison forced her to relinquish the chief executive title, she still collects upwards of $2 million a year in salary and bonuses from the firm.

Forbes' list of the 20 Richest Women in Entertainment is the subject of a one-hour special slated to air on the E! Entertainment Channel on Jan. 20. Check your local listings.

In Pictures: The 20 Richest Women In Entertainment

Methodology: Net worth estimates were arrived at by tallying the total earnings, including salaries, record sales, tours, merchandising and royalties over the course of a career. When relevant, we included real estate holdings, shares and other assets. A conservative rate of return was applied, less taxes and agent fees. The list is limited to those female celebrities active in their professional lives.