Background Primary care practices in the United States (US) are currently constrained in their ability to deliver high quality care due to population aging, insurance expansion, and an increasing prevalence of chronically ill patients. The nurse practitioner (NP) workforce plays a critical role in meeting the growing demands for primary care, particularly in rural and underserved areas. NPs are also more likely to deliver care to clinically and socially complex populations such as high-need high-cost (HNHC) patients. HNHC patients are adults who suffer from multiple chronic conditions and experience additional functional, behavioral, or socioeconomic needs. Despite comprising only 5% of the US population, HNHC patients account for nearly half of total health care expenditures and over 90% of Medicare expenditures. HNHC patients with behavioral health diagnoses such as depression or substance abuse face heightened challenges managing their conditions and consequentially have higher preventable spending and emergency department (ED) utilization compared to the overall HNHC population. Significant policy attention has been placed on enhancing primary care practices as a strategy to improve outcomes and reduce costs in HNHC patients. Structural capabilities are features of primary care practices (e.g., after-hours care or care coordination) which are needed to deliver high quality primary care and chronic disease management. Yet, to date little research has been done on structural capabilities in primary care practices where NPs deliver care to HNHC patients. The overall purpose of this dissertation is to understand how to enhance primary care delivery and structural capabilities to improve outcomes for HNHC patients. We have achieved the following specific aims: (1) Establish a clear definition of HNHC patients, (2) Identify existing primary care and payment models used among HNHC patients and evaluate their impact on ED utilization and costs, (3) Evaluate structural capabilities in NP primary care practices located in Health Professional Shortage Areas (HPSAs), and (4) Analyze the association between NP practice structural capabilities and ED utilization among HNHC patients with behavioral health conditions. Dissertation Chapters and Key Findings Chapter One includes an introduction to the landscape of current primary care delivery, the role of the NP workforce in expanding access, and the unique challenges of delivering care to HNHC patients. This chapter also discusses the conceptual framework guiding the dissertation, the specific aims of each study, and how each study will fill a gap in the literature. Chapter Two (Aim 1) consists of a concept analysis of HNHC patients using the Walker and Avant framework. Three subgroups of HNHC patients were identified: adults over the age of 65 who suffer from multiple chronic conditions with functional or behavioral health needs, the frail elderly, and patients under 65 years old with a serious mental health condition or disability. Antecedents that predispose an individual to becoming a HNHC patient include challenges accessing timely care, low socioeconomic status, or unmet needs. Persistent high spending occurs as a result of poorly managed chronic diseases leading to acute exacerbations, preventable health service utilization, and fragmented care between the acute and primary care settings. Chapter Three (Aim 2) is a systematic review of studies conducted from 2000-2020 on primary care and payment models used with HNHC patients. About half of the primary care models evaluated in the systematic review (11 out of 21 studies) showed no significant difference in ED utilization among HNHC patients. Care coordination and care management (15 out of 21 studies) demonstrated both positive and negative associations with ED utilization and costs. Primary care models that demonstrated significant reductions in ED utilization had shared features, including frequent follow-up, multidisciplinary team-based care, enhanced access, and care coordination. Chapter Four (Aim 3) includes a cross-sectional study of NP survey data from 2018-2019 on practice structural capabilities linked with data on primary care shortages (i.e., HPSA designation). Bivariate analyses and multivariable regression models were used to compare NP characteristics and structural capabilities in HSPA practices compared to non-HPSA practices. The majority of NPs in our sample (61%) delivered care in HPSA practices. NP practices located in HPSAs were significantly more likely to deliver care coordination compared to non-HPSA practices. We found no significant difference in prevalence of registries, after-hours care, or shared communication systems. Chapter Five (Aim 4) is a study of cross-sectional NP survey data from 2018-2019 on practice structural capabilities linked with Medicare Part A and Part B claims to identify HNHC patients and ED utilization. Multivariable Poisson models were used to estimate the association between ED utilization and structural capabilities in practices serving HNHC patients with behavioral health conditions including depression, alcohol use, and substance use disorder. Care coordination was associated with decreased rates of ED utilization among the overall HNHC population and those with alcohol use, but not among HNHC patients with depression or substance use disorders. Shared communication systems were associated with decreased rates of all-cause and preventable ED utilization among HNHC patients with alcohol use and substance use disorders. Chapter 6 is a summary of findings across studies in this dissertation and will present the strengths, limitations, and contributions to science. This chapter will also discuss implications for policy, practice, and directions for future research. Conclusion HNHC patients face complex and wide-ranging medical, social, and behavioral health needs resulting in poor clinical outcomes and high costs. Enhancing primary care is an urgent goal for policymakers to improve disease management while reducing overall costs of care. Findings from these studies demonstrate that NPs practice in underserved areas and are significantly more likely to deliver care coordination in HPSA practices and to HNHC patients with behavioral health conditions. Care coordination has the potential to increase effectiveness of primary care delivery by tailoring models to target specific HNHC patients. Shared communication systems also show promise for improving primary care delivery and reducing ED utilization among HNHC patients with alcohol use and substance use disorders. Future research should continue to explore how structural capabilities may enable NPs to deliver timely, high quality, cost-effective primary care for HNHC patients.