Linda Alvarado's mother didn't work--at least that's how she saw it. She didn't count taking in ironing from the big houses up the hill as a job. But ironing was work she never wanted her only daughter to do. "That was her gift to me," says Alvarado. "She did the housework so I could study."

Now, Alvarado jokes she may have learned that lesson too well. These days, her husband, a Colorado fast-food franchiser, oversees domestic duties while she dons a hard hat to prowl the site of Denver's expanding convention center. The project is just one of dozens her commercial contracting firm is building in several states. At 50, Alvarado is not only a self-made mogul in a field notoriously hostile to those of her gender; she's emblematic of a generation of women who have changed the face of the American workplace.

In 1952, the year she was born, barely 30 percent of women held a job. Now, they make up nearly half the U.S. labor force--an estimated 41 million wage earners. Like her, 6.2 million women own companies, contributing nearly $2.4 trillion to the economy. Others have stormed even the most stubborn male bastions, whether trucking or tech. That has transformed everything from the nation's labor laws to its notion of the family. "The speed of the change is astonishing," says Lynn Weiner, a dean at Chicago's Roosevelt University, who has written about the history of the female labor force.

The title of Weiner's study, From Working Girl to Working Mother, underscores the most striking aspect of that shift. Before World War II, working women were mainly single, fresh off the farm or an immigrant freighter, toiling as servants or factory hands. Now they're more likely to be harried supermoms, madly juggling business meetings with kids' day-care schedules.

In 1955, only 18 percent of mothers with children under age 6 worked. By 2001, that number had jumped to 64 percent--with about 2 million mothers of infants less than a year old holding down full-time jobs. As education and housing costs have soared, many women work not only because they want to but to keep their households afloat on the shifting tides of the American dream. By 1989, 80 percent of home buyers were two-income couples. And according to the U.S. Census Bureau, a family in the 1990s needed two paychecks to maintain the standard of living it had enjoyed two decades earlier. "Today," says Weiner, "most women work because they have to."

Not that the concept of the coprovider family is entirely new. Since the Pilgrims stepped off the Mayflower, women have toiled beside men, tending crops and trading goods under pressures that would make modern home-career conflicts look like a day at the spa. And as an exhibit called Enterprising Women, mounted by the Radcliffe Institute for Advanced Study, points out, women have run their own businesses at least since Mary Katherine Goddard, proprietor of the Maryland Journal, printed the first signed copy of the Declaration of Independence.

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