Since their introduction in the United States market, awareness, interest and use of electronic cigarettes (e-cigarettes) has steadily grown. E-cigarettes are battery-operated devices that produce an aerosol (vapor) instead of smoke. E-cigarettes are a rapidly changing technology. As the products and regulatory environment develop, so may the facts around their usage, health effects and public health impact. We will continue to monitor those developments and update this review to reflect the current state of knowledge.