Close relationships are integral to the health and adaptation of our species. The evidence is incontestable, and it is the building block upon which future theory, research, and practice rests. This foreword notes the distinct domains and independent histories of relationship science and health psychology and calls for further thorough integration within the broader context of team science. The articles in this special issue focus on interpersonal mechanisms, cultural specificity, personality and emotion regulation, couples dynamics in chronic disease, and the increasingly complex biological mechanisms involved in linking relationships to health outcomes. They point to the importance of life stage, especially childhood and late adulthood, for understanding unique relationship and health issues. The challenge remains to translate existing and future knowledge into interventions to improve social relationships for the benefit of physical and mental health.