Social scientists struggle on how to best operationalize and measure sexual orientation. Depending on the survey, researchers can use self-reports of lesbian, gay, or bisexual (LGB) identity, same-sex partner cohabitation, same-sex sexual attraction, or same-sex sexual behavior. All measures come with their own limitations. To illuminate differences in these measures, this study examines the intersections between self-reported sexual identity, attraction, and behavior among a nationally representative sample of US men and women aged 15–45. I explore how and when the three measures align, examine the determinants of self-identifying as gay or bisexual based on sexual behavior and attraction, and assess gender differences in the patterns. I find that about 20% of women and 10% of men aged 15–45 would comprise the LGB community if it were defined to include those who report at least one of the following: gay or bisexual identity, any same-sex attraction, or same-sex sex in the last year. This is much higher than the 6.4% of women and 3.6% of men aged 15–45 who self-identify as LGB. I conclude with recommendations that can aid in measurement of the LGB population, and discuss implications for using certain measures over others when conducting research on the LGB community.