PillSmart: Abstract Stimulant medications are efficacious in reducing impulsivity and other ADHD symptoms in youth, which is important because youth with ADHD have an increased risk of developing substance use disorders. Youth's non-adherence of stimulant medications is prevalent (~47-60%), and the misuse of prescribed stimu- lants is increasing among youth (~5-34%), which poses a serious public health concern. Without adequate monitoring support by parental caregivers, stimulant misuse and non-adherence in youth can increase the risk of substance use disorders, overdose, psychosis, and suicidality. There are no available evidence- based tools or interventions demonstrated to improve stimulant adherence and reduce misuse among youth with ADHD. Most adherence aids (e.g., medication reminder apps, 7-day pill organizers, smart pill caps) fail to ad- dress the potential misuse of stimulants as they cannot regulate medication access. Pill dispensers are effec- tive at improving medication safety and adherence, but they are designed for older adult/elderly patients, and they are not ideal for youth due to their large and heavy size. Our pilot data suggest that youth are concerned about stigma from others when using their medication, and the use of current dispensers could hinder adher- ence by potentially soliciting unwanted attention and triggering anticipated stigma (i.e., worry/concern about negative reactions from others). PillSmart™ is a primary prevention intervention that uses a novel, sleek, and discreet pill dispensing device (prototype developed) and mobile app (to be developed). PillSmart's pocket-sized pill dispenser aims to prevent misuse by securely storing medication and only dispensing the programmed dose at the scheduled time. PillSmart's interoperating mobile app aims to improve adherence and mitigate the parental burden by fa- cilitating remote supervision of the stimulant medication regimen. The overarching goal of this project is to de- velop PillSmart's system as an evidence-based prevention intervention to improve stimulant adherence in youth with ADHD, reduce the parental burden of stimulant medication management, and in the long- term, reduce the non-medical use of prescribed stimulants. This Phase I study will now develop the system's interoperability between the dispenser and the app, conduct usability testing of the system, and conduct a prospective single-arm clinical trial to determine feasibili- ty. We hypothesize that the PillSmart™ system will improve medication adherence, prevent non-medical use of stimulant medication, and reduce anticipated stigma in youth. A subsequent phase II study will conduct a multi- site randomized clinical trial among youth prescribed with stimulants and will integrate evidence-based behav- ioral approaches to determine whether PillSmart™ is an adjunctive intervention that improves medication ad- herence and reduces non-medical use.