Human Computer Interaction for Development (HCI4D) is a rapidly growing community that focuses on understanding and designing technologies for populations that are under-served, under-resourced, and under-represented. Current best practices for designing technologies in HCI4D tend to focus on only the needs of target users, however, technological interventions are often deployed in complex ecosystems that involve multiple groups of stakeholders who interrelate and influence one another. Further, HCI4D researchers often aspire to design technologies that effectively change people’s behavior, however, they rarely engage with foundational theories that are well known in the field of Behavioral Science. As a result, technological interventions in under-served communities do not work as intended and when they do, researchers struggle to achieve sustainability and scalability. This dissertation addresses these gaps by focusing on how to effectively design technologies that change the behavior of people in complex ecosystems prevalent in under-served communities. Through three case studies conducted in Kenya, New York, and India respectively, I demonstrate how (i) researchers can adopt an approach that engages multiple groups of stakeholders during the design process of a technological intervention and (ii) systematically draw on behavioral science theories to inform technology design. In the first case study, I focus on the design of a health feedback system for beneficiaries in rural Kenya; the second case study centers on the design of technologies for home health aides who support heart failure patients in New York; and the third case study, presents the design of a low-cost technique for researchers to reduce participant response bias in rural India. By applying the same approach across three highly diverse problem contexts, I show that it is generalizable across problem domains. In doing so, my dissertation makes multiple contributions to HCI4D and builds a stronger bridge between the fields of HCI4D and Behavioral Science.