In 1976, two years before the Alma Ata Declaration, the then Director-General of WHO, Dr. Halfdan Mahler, proclaimed a “social revolution in public health” [1]. He saw the establishment of the Special Programme for Research and Training in Tropical Diseases (TDR) Programme in 1975 as playing an active part in the emerging social and health development agenda. In TDR's first public document, Mahler, and his UNDP counterpart Morse stated: “Many millions of the people living in the tropical regions of the world are cut off from the mainstream of social and economic progress. Victims of a heavy burden of disease as well as of harsh economic circumstances, they are not free to choose and plan a better future… Health and development are therefore inextricably interlinked and any strategy for improvement must be based upon this reality” [2]. In these early years of “international” health, it became increasingly apparent that the development of new health technologies was only one component of improved healthcare. Research was required to support the delivery of those technologies, as well as to understand the barriers that could prevent or limit access to them. TDR was one of the first United Nations co-sponsored programmes to recognize this need. From 1977 onward, it had a dedicated focus on applied social and economic research and produced a body of evidence that promoted and expanded the theoretical underpinnings of this field, supported training in low- and middle-income countries where the needs existed, and developed a research agenda and funding to specifically address the “social and economic factors that affect the transmission and control of disease… and to increase the effectiveness of disease control programmes by integrating human behavioural (social, cultural, and economic) factors in programme conception, design, and management” [3], [4]. In its first phase, research investigated human behaviour related to disease exposure, disease-related beliefs and attitudes, attitudes to disease control, the social and economic impact of tropical diseases, costs and effectiveness of alternative disease control measures, and economic development and disease [5]. It focused on the development of theory and methods for applying the social sciences in public health, community participation in disease control, and the economics of tropical diseases [6]. A later emphasis on the “upstream” factors in infectious diseases and their control, from 2000 to 2007, drew from basic social sciences and their subdisciplines, e.g., (medical) anthropology, (health) economics, (medical) sociology, political sciences, and health policy research [7]. Research focused on equity and access, health sector reform, community participation, gender-sensitive interventions [8], [9], globalization, human rights [10], and ethics [11], [12]. TDR also defined and exemplified the concept of applied social sciences for public health (ASSPH) [13], [14] and called for increasing the numbers of scientists trained in these areas. It identified implementation research as a critical aid to help disease control programmes, citing an “absence of capacity and understanding in how to engage with communities and ensure their participation, and of the ability to adapt research methods and health technologies to local contexts,” concluding that “the uptake, effectiveness, and sustainability of these interventions remains limited” [15]. Later, TDR led efforts to define and establish operational and implementation research as scholarly fields [16], [17]. Operational research was defined as “research into strategies, interventions, tools, or knowledge that can enhance the quality, coverage, effectiveness, or performance” of health systems or disease programmes [18]. Implementation research pursues research into the delivery of efficient, sustainable, and effective services; appropriate structure of health systems; the policy process; and other components that are necessary to bring new and old control interventions into the routine practice of national health systems and improve access particularly for vulnerable population groups. Both types of research are complementary [19].