ABSTRACT: AIMS:To examine rules about smoking and vaping in the home in relation to beliefs about the relative harm of second-hand vapor (SHV) compared with second-hand smoke (SHS) in four countries: Canada, United States, England and Australia. DESIGN:Data were available from 12?294 adults (18+) who participated in the 2016 (wave 1) International Tobacco Control Four Country Smoking and Vaping (ITC 4CV1) Survey. PARTICIPANTS:All participants were current or recent former adult smokers. MEASUREMENTS:Data were analyzed by weighted logistic regression on rules about smoking and vaping in the home; odds ratios and 95% confidence intervals were reported, adjusted for demographic and behavioral variables. FINDINGS:Of all respondents, 37.4% allowed smoking inside their home. Among a subset who were current vapers (n = 6135), 60.4% allowed vaping in their homes. After controlling for demographic and behavioral characteristics, beliefs about the harm of SHV compared with SHS was not associated with allowing smoking in the home, but was associated with allowing vaping in the home [odds ratio (OR) = 2.86 in Canada, OR = 1.82 in the United States and OR = 1.68 in England]. Characteristics that were associated with rules about vaping inside the home included daily vaping (OR = 2.95, 2.04-4.26; OR = 7.00, 4.12-11.87; OR = 5.50, 3.40-8.88; OR = 7.78, 1.90-31.80), living with a spouse who vapes (OR = 2.48, 1.54-3.98; OR = 2.69, 1.42-5.11; OR = 4.67, 2.74-7.95; OR = 21.82, 2.16-220.9) and living with children aged under 18 years (OR = 0.50, 0.37-0.68; OR = 0.89, 0.48-1.65; OR = 0.76, 0.53-1.09; OR = 0.26, = 0.11-0.61) in Canada, the United States, England and Australia, respectively. Similar characteristics were associated with rules about smoking inside the home. CONCLUSIONS:Among current and former smokers in 2016 in Canada, the United States, England and Australia, 37.4% allowed smoking in the home; 60.4% of current vapers allowed vaping. Both concurrent users and exclusive vapers were more likely to allow vaping than smoking inside the home. Allowing vaping inside the home was correlated with the belief that second-hand vapor is less harmful than second-hand smoke.