Objectives: To gain a better understanding of electronic cigarette (e-cigarette) use behavior and experience among adult e-cigarette users, with the goal of informing development of future e-cigarette use measures. Methods: Between August and October 2014 six focus groups were conducted in Seattle. Participants (63% male; 60% >35 years old; 60% White): e-cigarette users who used combustible tobacco products either currently or in the past. E-cigarette discussion topics covered: their daily use pattern (eg, frequency), product-related characteristics (eg, nicotine levels), and perceptions about health risks and benefits. Results: Participants' descriptions of daily use were so varied that no common "unit" of a "session" easily summarized frequency or quantity of typical e-cigarette use. Most users had difficulty in tracking their own use. Participants reported nicotine craving relief when using e-cigarettes, but described e-cigarettes use as less satisfying than combustible cigarettes. Valued characteristics included "ready availability" and the possibility of using indoors. A unique aspect of the e-cigarette use experience is the option of adding flavors and having the ability to exhale "big clouds" of vapor/aerosol. Most perceived e-cigarettes as a better and safer alternative to conventional cigarettes, yet still sought further information about health consequences and safety of e-cigarettes from trusted sources. Conclusions: E-cigarettes users are far from homogeneous in their behavior and motivation for adopting e-cigarettes. A range of use patterns arising from both hedonic and utilitarian factors, along with product characteristics (eg, variable nicotine levels and flavors) extending beyond those of conventional cigarettes, suggest that new, specific e-cigarette use measures must be developed. Implications: The current study provides timely information on adult e-cigarette use behavior, which is a crucial step in measuring this new phenomenon and assessing the risks associated with using e-cigarette products. Our findings reveal that vaping is not a mere replacement for combustible cigarette smoking and that many users of e-cigarettes enjoy product characteristics such as flavors and "clouds" that are unavailable in combustible cigarettes. Therefore, commonly available cigarette smoking measures are not well suited to measurement of e-cigarette use behavior, and indication that new measures need to be developed to accurately track e-cigarette use.