Attention-Deficit/Hyperactivity Disorder (ADHD) is a chronic condition associated with substantial functional deficits as well as risk for poorer mental and physical health over the lifespan. Although ADHD symptoms often emerge during preschool, most children are not identified or treated during this developmental stage, missing the critical window in which early intervention may have the greatest impact on ADHD symptom trajectories. Additionally, even when evidence-based preschool behavioral interventions (behavioral parent training [BPT]) are delivered, treatment effects on core ADHD symptoms are inconsistent. One explanation is BPT's primary focus on ameliorating daytime impairment, whereas ADHD may be best understood as a 24-hour disorder with deficits in both daytime function and nighttime sleep. Indeed, sleep is frequently dysregulated in school-aged children with ADHD, is associated with increased ADHD morbidity (i.e., increased core ADHD symptom severity and comorbidity, poorer cognition and functioning), and when enhanced via behavioral sleep medicine (BSM), results in improved core ADHD symptoms. Our recent work has shown similar associations between sleep and ADHD symptoms in a primary-care based sample of preschoolers, suggesting that regulating sleep may represent a critical mechanism for early interventions seeking to alter ADHD symptom trajectories among preschoolers at risk. Recognizing the powerful connection between sleep and ADHD early in life, there is a critical need to adapt BPT to target behaviors across the 24-hour period in a single, efficient intervention via integration with BSM, the first-line intervention for sleep regulation in this age group. In addition, it is essential to optimize the delivery system to reach preschoolers at-risk for ADHD in real-world clinical care settings. For the proposed project, we will develop and evaluate the feasibility and acceptability of an 8-week telehealth intervention (Preschool Attention and Sleep Support; PASS) for preschoolers identified in primary care as at risk for ADHD (i.e., with elevated symptoms). PASS combines proven behavioral interventions to tackle daytime (via BPT) and nighttime (via BSM) ADHD impairments in a single, streamlined treatment. PASS will be delivered by behavioral health care providers and will guide caregivers in applying the antecedent-behavior- consequence (ABC) framework of BPT to reduce daytime behavior concerns and support sleep regulation. We will also evaluate the short-term effectiveness of PASS on core ADHD symptoms, functional outcomes, and comorbidity compared to BPT. Finally, we will examine whether PASS impacts the hypothesized target mechanism, sleep regulation via both actigraphy and caregiver report, and assess if improved sleep regulation is associated with reduced core ADHD symptoms. Findings from this study will provide the foundation for an adequately-powered RCT to evaluate the effectiveness of PASS to improve ADHD symptoms and sleep...