This Chapter describes methods for defining the level of damage from an event, its impact on the functional status of the affected society, and for identifying the needs that result from the damage. The event may produce damage to the population (injuries, death), its constructions and societal functions, and/or to the environment. Societal functions are described as production functions with inputs, transformations, consumption of resources, and outputs. Distinction between available supplies, requirements, and needs is stressed. The differences between damage assessments and needs assessments are described. Needs for goods and services that are greater than the available level of supplies produces deficits. The concept of thresholds of available resources required to maintain the pre-event functional state, those necessary to sustain basic functional states, and those required for survival is developed. Surplus of available supplies is subdivided into those needed for contingencies and those that comprise luxuries. Indicators are required that define either the level of function or the level of available supplies.Distribution of available supplies (who receives them) impacts whether critical and functional requirements of a population, or part of the affected population, are satisfied.When critical requirements are not met, the crude mortality rate increases. Severity scoring of the damage sustained and the effects of responses is described. All damage and needs assessments and responses to them relate to the pre-event state of the affected society, and the objective of disaster responses is to return the affected society to its pre-event functional state.