This chapter discusses some key concepts about disasters. Most disasters can be understood in three phases – namely pre-impact phase, impact phase, and post-impact phase. The pre-impact phase is the period of time before the onset of a disaster. The impact phase is the period of time during and immediately after a disaster, when rapid needs assessment and search-and-rescue relief work take place. The post-impact phase is the period of time after the impact of a disaster when the relief effort has reached equilibrium, or stabilised. Mega-disasters pose a great threat to the overall structure and functioning of society. The risk and impact of disaster at the individual and community levels must also be explored and assessed in order to guide effective disaster prevention and response protocol development. In a multidisciplinary subject like disaster and humanitarian assistance, proper classification of disasters may mitigate disaster risks and improve human well-being.