Background: Stop smoking services (SSSs), combined with pharmacotherapy, are more effective for quitting smoking than other aids used alone, including e-cigarettes. SSS uptake has nonetheless declined for six years. Amidst pressure on council budgets, this declining footfall has led to many areas reducing or discontinuing services. When making cuts, councils often cite widespread vaping and suggest this indicates smokers nowadays need SSSs less. This PhD aimed to investigate whether vaping amongst smokers can influence SSS use and, if so, how this occurs. Methods: A systematic review synthesised evidence on sociodemographic differences in e-cigarette use from 58 studies identified across seven databases. Repeat crosssectional survey data was collected from 2,139 current smokers through questions added into the ‘Smoking Toolkit’ survey. Multivariable logistic regression assessed associations between SSS uptake and: a) e-cigarette use; b) knowledge/belief statements about e-cigarettes or SSSs. Finally, 46 semi-structured interviews were undertaken with smokers and SSS professionals at three sites. Principles of framework analysis were applied to examine factors influencing smokers’ decisions to use ecigarettes or SSSs, including potential impacts of differences in perspectives between smokers and SSS professionals. Findings: Systematic review results suggested higher e-cigarette use among younger adults, males and people of white ethnicity. Survey data showed smokers who vaped were more likely than others to report both past and planned SSS uptake. Further analyses showed beliefs about e-cigarettes’ effectiveness and familiarity with vaping were associated with decisions to use SSSs. Semi-structured interviews indicated a range of views on potential risks from vaping. These appeared to be key factors influencing – for smokers – their e-cigarette use, and – for services – the support provided in relation to e-cigarettes. Conclusion: Smokers using e-cigarettes remain keen to use SSSs. Given social gradients in smoking rates, removing opportunities to access such services may have retrograde health impacts for society’s most vulnerable people.