Defining who works in STEM has relied on top-down categorizations of occupations. This study takes a bottom-up approach and surveys a national sample of workers to classify their jobs. We identify a sizeable group of workers who are in the “periphery STEM workforce,” who report working in STEM jobs, but whose occupations are outside of STEM classifications. Using this approach, the gender gap in STEM workforce participation decreases substantially because women are more likely to work in the periphery. However, women in the periphery are compensated less than men, a fact invisible using the current classifications.