Early life experiences strongly shape developmental trajectories in a wide range of domains, such as mental, emotional and behavioral health, education, and employment. Children living in poverty face disproportionate adversities starting from infancy. Home visiting has become a widely used service approach nationwide in promoting maternal and infant health and early childhood development. It has been increasingly acknowledged that home visiting is a multidimensional and complex process, that interventions rarely conform precisely to what is expected, and that considerable variability exists in the quantity and quality of services and in the characteristics and needs of families receiving services. Therefore, despite the effectiveness established through randomized controlled trials, newer generations of research have increasingly focused on how home visitation works, for whom it works, and under which contexts it works. This three-paper dissertation used data from mothers who were assigned to the intervention group in a randomized controlled trial of doula-enhanced home visiting services. It explored the variation in the service implementation process and associated mother and family characteristics, and the causal role of the mother-doula relationship in promoting positive parenting. This dissertation found that: (1) There were differential patterns of supportive relationships across mothers, and that there was evidence for both “relationship-driven” and “needs-driven” mechanisms explaining mothers’ involvement with their service providers; (2) Half of mothers remained in services at one year postpartum, and the largest percentage of attrition occurred from the second to the third month postpartum. Mothers who reported higher levels of risk-taking behavior and were at educational risk, who used only English at home, and who were in later months of pregnancy at service initiation discontinued services more rapidly; (3) A very positive mother-doula relationship was associated with greater maternal sensitivity and careful handling of the infant, but not associated with maternal intrusiveness, detachment or stimulation. This dissertation revealed an important variability in the quantity and quality of services received by home visiting participants and demonstrated the importance of strong mother-home visitor relationships. Ongoing supportive and reflective supervision, peer support, and training for home visitors are critical in encouraging them to continue engaging high risk mothers and help them be gently persistent in their efforts to build trust with mothers. As family demographics become more diverse in the U.S., home visiting programs need to train their workforces to meet the needs of families facing different challenges and adversities. A continued research interest in the service delivery process and active ingredients is needed for successful intervention programs.