Marissa Mayer Jemal Countess / Getty Images / Time Inc. Then vice president of consumer products for Google Marissa Mayer speaks onstage at the FORTUNE Most Powerful Women Dinner New York City at Hudson Room at the Time Warner Center on May 24, 2011 in New York City. Marissa Mayer's appointment as CEO of Yahoo! was an exciting development for her legion of fans - both male and female. It was also an undeniable cause for celebration among those who would like to see more women in positions of power, not only in Silicon Valley, but throughout corporate America. In 2012, it's hard to believe that only 19 companies out of the Fortune 500 are led by women. The tech industry has made somewhat more progress than other sectors - at least at the very highest levels - as the accomplished and inspiring woman on this list demonstrate. But there's still a troubling lack of women further down the corporate ranks. Claire Cain Miller of The New York Times recently described the challenges that women continue to face in the technology world. There remain distressingly few women among Silicon Valley engineers, start-up entrepreneurs, venture capitalists and computer science and engineering majors, for reasons including the technology industry's girl-repelling image problem, the tiny number of powerful women role models and the insular Silicon Valley deal-making boys" club. One hopes for a day not too far away when the appointment of a woman - yes, even an expectant mother - as CEO of a major American company is noteworthy not for gender, but for the executive's experience, accomplishments and track record of achievement. Hopefully one day soon lists like this one will no longer be necessary. But until then here is a collection of the most influential women in technology, led off by Mayer herself. More: Can Google Star Marissa Mayer Save Yahoo!? Even before she was named CEO of Yahoo!, Mayer was one of the most respected executives in Silicon Valley, but her appointment to lead the purple-hued Internet pioneer instantly makes her one of the most prominent female chief executives in the world. On Monday, shortly after her appointment was announced, Mayer - a 37-year-old self-proclaimed "geek," and Google's 2oth employee - revealed to Fortune that she is pregnant with her first child, due in October. An accomplished engineer with a keen eye for design, Mayer has built a reputation as a brilliant, intense, and tireless executive with a passion for improving "the user experience." In a classic example of her geekiness, Mayer famously informed a reporter several years ago that she was one billion seconds old (31.7 years). Yahoo! disclosed Thursday that Mayer will receive a base salary of $1 million, with the potential for $15 million if she stays for 5 years. It's unlikely Mayer joined Yahoo! for the money, however. Thanks to stock options she received as one of Google's earliest employees, Mayer is worth an estimated $300 million. Earlier this year, she joined the board of directors at Walmart. A Wausau, Wisconsin native, Mayer joined Google in 1999 after earning undergraduate and graduate degrees at Stanford University, where she specialized in artificial intelligence. Mayer was Google's first female engineer, and quickly took the helm of the company's user interface team, where she was instrumental in developing Google's iconic search box layout. She would eventually become responsible for many of Google's most successful consumer-facing products, including Gmail, Google News, and Google Maps. Industry observers greeted Mayer's appointment as Yahoo! CEO warmly, but the struggling tech giant faces a steep climb to return to its former glory. MORE: Marissa Mayer: Irrelevant "Superwoman" or Agent of Change? Next: Sheryl Sandberg Sheryl Sandberg Chris Ratcliffe / Bloomberg / Getty Images Sheryl Sandberg, chief operating officer of Facebook Inc., speaks during a session on day two of the World Economic Forum (WEF) in Davos, Switzerland, on Jan. 26, 2012. Longtime Google executive Sheryl Sandberg joined Facebook as Chief Operating Officer in 2009, and quickly established herself as one of the most powerful voices in Silicon Valley. Prior to joining Google, Sandberg was chief of staff for Clinton-era Treasury Secretary Larry Summers, who also happened to be one of Sandberg's professors at Harvard. (Sandberg won the John H. Williams Prize for the top graduating student in economics. She would also earn an MBA from Harvard.) Sandberg would later introduce Summers to Marc Andreessen, the billionaire co-founder of Netscape who now runs Andreessen Horowitz, one of the most influential venture capital firms in Silicon Valley. Summers now serves as a special adviser to AH; Andreessen is on Facebook's board of directors. Thanks to her Facebook stock options, Sandberg is estimated to be worth over $1 billion. MORE: 5 Ways Women Can Defy the Odds & Start a Business As COO of Facebook, Sandberg is widely credited with bringing a healthy dose of "adult supervision" to the company, which resembled a frat house in its early years. Over the last few years, Sandberg has become an inspirational role-model for her advice that young women should "lean in" to their careers. "Put your foot on that gas pedal and keep it there until the day you have to make a decision, and then make a decision," Sandberg told students during her 2011 commencement speech at Barnard. In addition to Facebook, (where she was recently appointed to the board of directors), Sandberg serves on the board of the Walt Disney Company, Starbucks, Women for Women International, the Center for Global Development and V-Day. Earlier this year, Sandberg was named to the Time 100, the annual list of the 100 most influential people in the world. Next: Virginia Rometty