Security assurance of infrastructural systems such as electric power grids against regular incidents,which havelow-impact and high probability, has been concerned by designers and operators from the past.However, performance of these systems is still severely disturbed by extreme events.Accordingly, it isin essence to study the system behaviorduring severeincidents in both design and operation stages and take required considerations to overcome shortcomings if needed. This behavior is known as a new feature referred to as resilience of an infrastructural system.Resilience denotes thetemporal performance including robustness, vulnerability, and recoverability of a system in a severe disturbance condition.This paper explains the concepts and fundamentals of resilience assessment and clarifies the main differences of this notion with more well-known concepts such as security and reliability.Due to the increased incidence rate of natural events, which are categorized within high impact and low probability incidents, and the disastrous impacts of even short blackouts, resilience studies should be included as a necessity in the operational plans of respective utilities.

Keywords: Resiliency, High-Impact Low-Probability (HILP) Events, Robustness, Recoverability

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