Disasters can be defined as basic disruptions of the social context within which individuals and groups function, or a radical departure from the pattern of normal expectation. In this paper the subsequent phases of disasters _ preimpact or threat, warning, impact, recoil and postimpact according to most authors _ are analysed in this respect in a review of the literature. Although difficult to assess and monitor, psychosocial processes appear of tremendous importance in disasters. They are the determinants of community preparedness, explaining for example the effectiveness of the authorities in reaching the community during the warming phase.