Electronic cigarettes (e-cigarettes) and individual-level interventions may either exacerbate or reduce inequalities in smoking cessation depending on differential effectiveness according to socio-economic position (SEP). This thesis addressed three distinct but complementary objectives across five studies. First, study 1 involved a systematic review to examine whether interventions tailored for disadvantaged SEP moderated effectiveness for smoking cessation compared with non-SEP-tailored approaches. SEP-tailored interventions were no more effective than non-SEP-tailored interventions for smoking cessation among disadvantaged smokers. Second, studies 2 and 3 modelled population-level trends in e-cigarette usage by SEP. Study 2 found that from 2014 to 2017 in England, e-cigarette use i) was greater among advantaged smokers compared with lower SEP smokers but this difference lessened over time, ii) was similar during a quit attempt across SEP groups, and iii) increased among all long-term ex-smokers but was more common among more disadvantaged ex-smokers. Study 3 examined the long-term ex-smoker sub-group from 2014 to 2019 in greater detail. Up until 2019 e-cigarette use continued to increase and diverge by SEP, with greater use among more disadvantaged groups. Separate analyses of ex-smokers who quit smoking before 2011 found a small increase in e-cigarette use over time but no evidence of differences by SEP. The final, qualitative objective explored what lower SEP individuals thought about e-cigarettes for smoking cessation. Study 4 highlighted that participants were using an e-cigarette in their otherwise unsupported quit attempt for health reasons, to save money, for family, and for enjoyment. Study 5 participants found an individual-level smoking cessation intervention involving the offer of an e-cigarette to be acceptable, and highlighted barriers and enablers to participation. This thesis highlighted that SEP-tailored and non-SEP-tailored individual-level interventions are not currently equity positive, and that e-cigarettes could provide a route out of smoking for more disadvantaged and dependent smokers who cannot quit otherwise
