Over the past three decades, the overall percentage of women receiving degrees in science, technology, engineering, and mathematics--known collectively as the STEM disciplines--has increased dramatically. This growth tends to mask at least three other aspects of the demographics of the science and technology workforce. Unfortunately, aggregated data mask the attrition of women at every phase of the educational and career STEM pipeline. Despite grades and other academic attainments equal to or surpassing those of the men who remain in STEM fields, more women than men leave science and engineering. Why do women exit the STEM workforce? The answer is not genetic disposition or lack of interest. For both male and female scientists, marriage and family create demands that can cut short a thriving STEM career. Yet another major source of leakage in the pipeline results from lack of networking and mentoring. Many institutions of higher education are reviewing and reforming their policies and practices in response to the national focus on women's participation in science and shortages in the science and technology workforce resulting from national security measures introduced after September 11, 2001, which have made it difficult for highly skilled non-U.S. workers to get U.S. visas. Attracting women to science and high-tech entrepreneurship and then retaining them will require changing the culture of science to make it more family-friendly and inviting.