Objectives: To examine young adults' knowledge of e-cigarette constituents and regulation and its association with product use and self-reported exposure to marketing. Methods: Young adults (18-34 years, N = 1,247) from a U.S. web panel were surveyed in March 2014. Using multinomial logistic regressions, self-reported exposure to marketing was examined as a predictor of whether participants responded correctly (reference category), incorrectly, or "don't know" to four knowledge items-whether e-cigarettes contain nicotine, contain toxic chemicals, are regulated by government for safety, and are regulated for use as a cessation aid. Analyses adjusted for demographics and smoking status and were weighted to match the U.S. young adult population. Results: Most respondents did not know if e-cigarettes, contain toxic chemicals (48%), are regulated for safety (61%), and are regulated as cessation aids (68%); fewer than 37% answered all of these items correctly. Current users of e-cigarettes (past 30 days) had a lower likelihood of being incorrect about safety testing (p = .006) and being regulated as a cessation aid (p = .017). Higher exposure to e-cigarette marketing was associated with a lower likelihood of responding "don't know" than being correct, and with a higher likelihood of being incorrect as opposed to correct about e-cigarettes containing nicotine. Conclusions: Knowledge about e-cigarette constituents and regulation was low among young adults, who are the largest consumer group for these products. Interventions, such as warning labels or information campaigns, may be necessary to educate and correct misinformation about these products.