In many ways warning can be the most important phase of the disaster response. Warning is thought of not just in terms of mechanical devices but in terms of psychological structures and processes. Warning is not only Advance notification of the existence of danger but also information about what can be done to prevent, avoid, or minimize the danger. The characteristics of the disaster agent - frequency, speed of onset, scope of impact, destructive potential, etc. -- affect the warning process. Before a warning message can be issued, threat data must be collected, collated, and evaluated. This report examines what is involved in these processes. Multiple organizations are frequently involved in collecting data; thus it must be compiled. In order for this information to be useful it must be evaluated. The decision to warn and the dissemination of the message are discussed: Who is to be warned, when, how, what the message includes. The response to warnings of danger is also considered. Included among the factors influencing response are the sociocultural framework, the historical setting, and the immediate ongoing social situation. The report concludes with a discussion of implications for nuclear catastrophe.