Abstract: Defining who works in STEM has traditionally relied on top-down categorizations of occupations, typically based on predetermined occupational coding schemes. This study takes a bottom-up approach and directly surveys a national sample of workers in the United States to classify their jobs based on their roles and tasks they perform when on the job. With this bottom-up approach, we identify a sizeable group of workers who are in the ‘periphery STEM workforce’ who report working in STEM jobs but whose occupations fall outside of top-down STEM classifications. When including the periphery STEM workforce as part of the broader STEM workforce, the gender gap in STEM workforce participation decreases substantially because women are more likely to work in the periphery. However, women working in the periphery are compensated less than men, a fact that remains invisible when using current top-down classification schemes.