“Health system” is a term generally considered to be relatively recent. It is defined as all organizations, institutions, and resources that produce actions whose primary purpose is to improve health, whether these be targeted at individuals (such as health-care delivery) or populations (such as public health measures). Health-care and public health institutions have a long history, but the notion of an organized “health system” is a relatively recent development (dating from the mid-20th century). In low- and middle-income countries (LMICs), Western medicine was often introduced by former colonial authorities through the construction of public hospitals, health centers, and training schools, with church authorities also making a major contribution. As in high-income countries, there was a gradual process over the latter half 20th century to construct an organized and coordinated national health system. However, health systems became a key focus of international attention only in the late 1990s, when it became apparent that achieving the health-related Millennium Development Goals (e.g., reduction of child and maternal mortality; control of HIV, TB, and malaria) was threatened less by the availability of technical solutions and more by the ability of health systems to put them into practice. More recently, the Ebola epidemic in West Africa highlighted the critical importance of health systems in ensuring health security. In response to the increased awareness of the role of health systems, significant attention has been paid to defining the health system and its goals, categorizing its elements, assessing problems and testing solutions, and seeking to identify the relationship between different health system configurations and overall performance. Over time, specific issues within the general area of health systems have received special attention, including achieving universal health coverage (where the whole population of a country has access to health care and protection against its costs), the role of primary health care, the relative merits of different ways of financing a health system, the relative roles of public and private health sectors, and the appropriate mix of different types of health worker. Many disciplines can contribute to improved understanding of health systems, including economics, sociology, anthropology, history, political science, and management science. Until recently, the discipline of economics has tended to dominate the study of health systems. However, with the emergence of health policy and systems research as an important area of study, other disciplines have been making growing contributions, especially political science and the behavioral sciences concerned with the behavior of both individuals and organizations.