Throughout this report we have been considering the "normal" or everyday problems of spaceflight. In this chapter we turn to the special problems arising from unusual and threatening events which we label "crises." For present purposes, a crisis may be defined as any situation which arises over a relatively short period of time and which could result in severe or even life-threatening consequences. Crises can arise either from physical events external to the crew, such as a meteor strike or an equipment malfunction, or from internal psychosocial causes, such as a severe psychological disturbance on-board. We will consider some of the situations which could place the crew under sudden and extreme stress, examine the research that relates to these issues, suggest ways in which these events might be avoided or dealt with, and specify those research questions which suggest themselves in coping with potential crises in future spaceflight.