Active faults that extend to near the Earth's surface may pose a direct threat to the safety of engineered structures. Usually the threat can be mitigated by taking appropriate siting or design actions. Such actions require a thorough understanding of the characteristics and magnitude of the faulting hazard based on detailed field investigations of the site locality and an adequate assessment of the characteristics of past faulting in the site's tectonic regime. An active fault may be defined in purely geological terms as one that has exhibited displacement in the present tectonic regime. However, for engineering considerations an active fault is usually defined in terms of a specified time of most recent movement, or a longer time in which multiple movements have taken place. Deterministic or probabilistic approaches are used to assess faulting hazard for the purpose of engineering design consideration. Support is building for the use of probabilistic procedures alone or combined with deterministic procedures to assess faulting hazard for design of critical facilities.