Missiology, or mission science, is the area of practical theology which studies the mandate, message and work of the Christian missionary. Missiology is a multi-disciplinary and cross-cultural reflection on all aspects of the propagation of the Christian faith, embracing theology, anthropology, history, geography, theories and methods of communication, comparative religion, Christian apologetics, methodology, and inter-denominational relations. Because mission science considers both the positive and the negative consequences, as well as the strategies of the spread of Christianity, missiology also touches on the environmental impact of evangelization and charitable work, including practical facets of international politics and economic development. One of its most difficult challenges is to distinguish between Christian practices that are essential to Christianity and therefore must be practiced by Christians in all cultures, and other strictly cultural expressions of Christianity that can be changed and adapted to a different culture.

Mission science more commonly refers to the special, scientific objectives of a multi-purpose errand, for example, of a space flight.