As a desired outcome of education, ethical thinking has roots in philosophy, developmental psychology, and political movements that advocate for developing knowledge, capabilities, and dispositions beyond those associated with traditional school subjects. Economists, business leaders, and educators have long advocated for preparing students for an increasingly global, interconnected, and technologically complex world, where individuals and groups are brought into dialogue on issues that necessarily invoke notions of justice, rights, the good, and other ethical considerations. This paper proposes a definition of ethical thinking as a 21st-century skill, drawing on K-12 and higher education trans-academic skills frameworks that foreground ethics, responsibility, and related competencies. Literature in philosophy, moral psychology, and education are brought together to inform that definition and to describe theories on the development, malleability, and cultural variability of moral reasoning, the most researched construct related to ethical thinking. The paper describes and presents evidence supporting various instructional approaches to moral development, particularly its cognitive aspects. The paper concludes with a survey of assessments of moral reasoning and key considerations in assessing ethical thinking.