Electronic nicotine delivery systems, also called e-cigarettes, are devices that vapourize liquid, typically comprising nicotine, propylene glycol, glycerine and flavourings. Switching from smoking tobacco cigarettes to using e-cigarettes --known as vaping--may reduce user harm, by supporting quitting or acting as a lower risk substitute. However, the degree of harm reduction is uncertain. Governments that are considering policies to restrict vaping should consider the optimal regulation of e-cigarette products, including defining where vaping may occur. Here, we explore some of the arguments for and against extending indoor smoke-free laws to also cover vaping. Arguments for vaping First, allowing vaping in indoor public places may encourage smokers to switch to vaping, by making it relatively more attractive as vaping would be allowed where tobacco smoking is not. Some e-cigarette users have voiced this potential benefit of normalization of vaping when arguing against any bans on public vaping. (1) Nevertheless, we are not aware of any clear evidence supporting this argument as an important driver for smokers switching to vaping. Other factors, such as health reasons or the lower cost of vaping, seem to be more important for switching from smoking to vaping. Furthermore, if vaping indoors does actually normalize vaping for smokers, then logic would suggest it might also normalize vaping for non-smokers. Second, allowing vaping in indoor public places where smoking is not permitted could minimize any discomfort that e-cigarette users may experience from nicotine withdrawal when being in such settings. However, evidence suggests that this discomfort is fairly modest. For example, in a survey conducted among exclusive e-cigarette users in the United States of America, only 12% (124 of 1034) reported finding it difficult to refrain from vaping in places where they were not supposed to. (2) Arguments for prohibiting vaping First, at a distance, smoking and vaping may look similar to some people, since both activities produce visible clouds exhaled from people's mouths after they have drawn on a cigarette or device. Some e-cigarette users admit to this similarity, e.g. some cite visual similarity as a reason why they do not vape around people who are eating. (1) Given such similarities, permitting indoor vaping might renormalize tobacco smoking in smoke-free indoor environments and may lead smokers to query: if vaping is permitted, why is smoking not allowed. Renormalization of tobacco smoking would be particularly problematic if it increases the risk that children become susceptible to or initiate smoking. Indeed, some research suggests that children may misperceive vaping as smoking. (3) Nevertheless, the authors of this study speculated that once these products are more common and the purpose of them is known, seeing people use them should normalize quitting behaviour. (3) A second argument is that close exposure to vaping among people who have recently quit smoking or vaping might trigger them to relapse to smoking. For example, an experimental study among young-adult tobacco smokers reported that exposure to a video showing vaping significantly increased their urge to smoke as well as their desire for tobacco cigarettes and e-cigarettes. (4) Similarly, another experimental study found that exposure to the e-cigarette cue but not the tobacco cigarette cue also significantly increased desire to smoke an e-cigarette. (5) Evidence suggests that many smokers support smoke-free areas, because this helps encourage them to quit. (6) It seems plausible that this reasoning would also apply to e-cigarette users, who wish to either constrain the level of their vaping or to quit vaping and may therefore favour indoor areas being vape-free. Third, passive exposure to e-cigarette vapour might lead to adverse health effects according to a systematic review of 16 studies. …