Abstract Purpose Youth in the United States (US) are using electronic nicotine delivery systems (ENDS) at a high rate. Modifications to ENDS by youth can introduce additional health hazards which have not been previously considered. To better understand these risks, we need more information on what these modifications are, the motivations behind them, and the sources of information on modifications. Methods Utilizing a trained moderator, in 2020-2021, we conducted one-on-one interviews with 19 youth ENDS users aged 16-17 living in the US and analyzed their responses using a qualitative description approach. Results The most prominent modification was to the e-liquid; youth indicated they mixed e-juices to create new flavors and added substances not intended for vaping, including illicit drugs such as cannabis and cocaine. Few youths from our sample were interested in achieving a specific nicotine level to vape, and modifications to the battery and coil/wick were less frequently mentioned. Some of these modifications were motivated by a desire to achieve specific experiences with their device. At other times, modifications were made due to necessity because of limited access to ENDS devices and supplies. YouTube and peers were the main sources of information about modifying. Conclusions Youth are making modifications that are both intended and unintended by the manufacturer. Adding illicit drugs and other substances not made for vaping is of particular concern. Understanding how youth modify ENDS and how that changes their use is important to guide regulatory policy intended to reduce harm to youth from ENDS use. Implications Youth from our study indicated that they make modifications to the ENDS devices, specifically to the e-liquid. These modifications are both intended by the manufacturer, such as changing the e-liquid and replacing coils, and unintended, such as adding substances not meant for vaping. Future policies aimed at reducing youth ENDS use should consider mandating better safeguards against modifications that appeal to youth.