The aims of the study were to (1) describe how and why smokers start to vape and what products they use; (2) relate findings to the COM-B theory of behaviour change (three conditions are necessary for behaviour change (B): capability (C), opportunity (O), and motivation (M)); and (3) to consider implications for e-cigarette policy research. Semi-structured interviews (n = 30) were conducted in London, UK, with smokers or ex-smokers who were currently using or had used e-cigarettes. E-cigarette initiation (behaviour) was facilitated by: capability (physical capability to use an e-cigarette and psychological capability to understand that using e-cigarettes was less harmful than smoking); opportunity (physical opportunity to access e-cigarettes in shops, at a lower cost than cigarettes, and to vape in “smoke-free” environments, as well as social opportunity to vape with friends and family); and motivation (automatic motivation including curiosity, and reflective motivation, including self-conscious decision-making processes related to perceived health benefits). The application of the COM-B model identified multiple factors that may lead to e-cigarette initiation, including those that could be influenced by policy, such as price relative to cigarettes and use in smoke-free environments. The effects of these policies on initiation should be further investigated along with the possible moderating/mediating effects of social support.