This chapter provides an overview of the chemical contents, emissions, and factors influencing the chemicals found in both the e-liquid and the aerosol of electronic nicotine delivery systems (ENDS). A section of the chapter examines the chemical composition of ENDS, detailing the various additives, flavorings, and other substances commonly found in e-liquids, as well as exploring the diversity of ingredients used. A discussion on the emissions produced during the use of ENDS, focusing on the formation of toxicants and other harmful compounds in the aerosol, is also presented. It covers the factors affecting the emissions generated, including the effects of temperature, device type, and e-liquid composition. By understanding these processes, readers will gain insights into the complexities and dangers associated with ENDS use.