Abstract: Early warning is one of the most important physical concepts in the reduction of natural disasters for modern society. Prediction/forecast face intrinsic difficulties in physics, so early warning plays a most critical role in practical applications for sustainability. We review the basic concepts and physics of early warning, including the difference between prediction/forecast and early warning, front detection early warning and on-site early warning, and the common physical problems of early warning systems in different fields-finite propagation, the ripple effect, and scaling, as well as the main technical requirements of practical early warning systems.