SUMMARY/ ABSTRACT Project 3's long-term objectives are to understand the substitutability between novel and more frequently used tobacco products, the factors promoting and mitigating illegal tobacco purchases, and the generality of these findings in diverse non-laboratory samples in different types of users. Project 3's aims are 1) to compare the substitution between different tobacco/nicotine products as a function of tobacco-user type, 2) to model policies that may promote and mitigate illegal tobacco purchases for different types of tobacco users, and 3) to examine the generality of laboratory-based findings and their moderators using diverse samples (US, England, and Canada) from the International Tobacco Control Policy Evaluation Project. Participants will be both exclusive cigarette smokers and dual cigarette/nicotine vaping product users because these individuals are most at risk for negative health outcomes associated with combusted tobacco. The methods will employ the Experimental Tobacco Marketplace (ETM), an online purchasing experience that places a mix of products, prices, and specific regulations under experimental conditions that simulate “real world” circumstances. The ETM provides estimates of the effects of product changes and/or policy changes on tobacco purchasing patterns, including between-product substitution in different types of tobacco product users. Here the ETM will be used to examine the effects of novel products, such as heated tobacco and nicotine pouch products, on purchasing patterns and the uptake of different products (substitution). Further, the Illegal Experimental Tobacco Marketplace (iETM), a methodological extension of the ETM, was developed to examine conditions that promote or mitigate against illegal purchases. Here the iETM will be used to examine the effects of regulatory environments that tend to promote illegal purchases, such as high cigarette prices and reduced nicotine content cigarettes. Moreover, this project will examine policy strategies that may mitigate illegal purchases, such as health communications, availability of nicotine replacement therapy, and penalties. The ultimate health impact of this project is to inform tobacco regulation, reduce tobacco-related harm, and improve public health.