Smartphone mobile health technology has the potential to empower patients to take an interest in their wellbeing and contribute to their own health care. However, this technology centers on the critical factors of demonstrated efficacy, usability, and sustained engagement when using apps. This integrative review presents a synthesis of the current research addressing the usability, engagement, and clinical outcomes of mobile health apps across various patient populations. The synthesis of findings identifies current understanding of the impact of apps on varied health outcomes, associated gaps in knowledge, and directions for future research. Conclusions from this review support the novelty of smartphone mobile health apps, but also highlight the significant dearth of published research. Evidence of improved clinical outcomes has not yet been established. Therefore, there is a critical need to conduct rigorous trials to expand upon the favorable short-term results of the reviewed studies.