The prevalence of the use of electronic nicotine delivery systems (ENDS) has increased rapidly in the past few years. In the absence of clear negative health messages, the decision of adults on whether or not to initiate ENDS use is influenced by various factors, including the pervasive promotion of ENDS as being effective in smoking cessation and a safe alternative to tobacco, which are unsubstantiated claims. The appeal of advertisements promoting ENDS as an enjoyable lifestyle choice and a high-tech product influences the choice of young people. Thus, the most common reasons for ENDS use in adults are its perceived safety and its efficacy as a cigarette cessation aid. Curiosity, flavors, and peer influence have been identified as the top reasons for ENDS use in adolescents and young people. Although ENDS are generally considered as a single product class, the systems constitute a diverse group with differences in the production and delivery of the various agents. The composition of the aerosol generated by ENDS depends on several factors, including, the electronic liquid constituents, the ENDS features, and user behavior. Currently available data indicate that ENDS aerosols are not harmless, especially with respect to body systems that are sensitive to various toxic effects. The literature on the effects of ENDS on the various body systems is sparse and marked by a lack of standardization in methods. The long-term effects of ENDS are as yet unknown. Studies of short-term exposure to ENDS aerosol document several biological and functional effects on the respiratory, cardiovascular, immune, and central nervous systems. At least under certain conditions, ENDS have been shown to deliver physiologically active quantities of nicotine, and can thus produce and/ or maintain nicotine dependence. The current evidence is sufficient to justify cautioning pregnant women, women of reproductive age, children and adolescents about the hazards of ENDS use, because of the potential for fetal and adolescent nicotine exposure to have long-term consequences for brain development. Passive exposure to ENDS aerosol has not been well studied because ENDS are relatively new products, but it is of concern, because of its potential adverse health effects for people who are involuntarily exposed. The majority of relevant studies concluded that passive exposure to ENDS aerosol may indeed pose a health risk. The evidence for the effectiveness of ENDS as a method for quitting or reducing tobacco smoking is limited and of low quality. Some studies demonstrated a significant relationship between ENDS use and increases in smoking cessation, while others found no association or a negative one. The findings of prospective studies on adolescent and young adult populations suggest that ENDS use is a clear and consistent indicator of the likelihood of subsequent initiation of cigarette and other combustible tobacco product use, at ages spanning from early adolescence through emerging adulthood.