System dynamics, a core methodology in systems thinking, was developed in the late 1950s. Systems thinking is one of the competencies in public health, and students in graduate schools of public health are expected to learn how to apply its tools to address public health problems. A series of articles introducing systems thinking were published in the American Journal of Public Health in 2006. In 2009, the World Health Organization (WHO) published a report on strengthening health systems through systems thinking. Health inequality is one of the most serious public health problems and should be addressed using a systems thinking approach.