Electronic cigarettes (e-cigarettes) have proliferated at a rapid rate since their introduction into the US market in 2007 and their use as a form of nicotine delivery far outpaced the science base. Important questions revolved around their abuse liability and safety. Although the design of these devices continues to evolve, the investigators have previously described nicotine intake, systemic retention, pharmacokinetics, and effects, as well as vaping behavior and self-administration of e-cigarettes. The investigators showed that e-cigarettes deliver as much nicotine from 15 puffs as a typical tobacco cigarette (~1 mg), most of which is systemically retained, and the shape of the plasma nicotine concentration-time curve is also similar to tobacco cigarettes, except that the maximum plasma nicotine concentration is, on average, lower for e-cigarettes. During ad libitum access, e-cigarettes were vaped intermittently in groups of 2-5 puffs or single puffs such that plasma nicotine levels rose gradually and peaked at the end of the 90-minute session. This differs from the rapid increase in plasma nicotine observed during controlled use of e-cigarettes or during tobacco cigarette smoking. Taken together, these results indicate that e-cigarettes have the potential to produce and sustain nicotine addiction but their use and abuse liability may differ from tobacco cigarette.