Energy security exists in an energy system until an event occurs which increases the stress on one or more of its entities. A resilient entity, designed to recover quickly from an event, will return the system (and, by extension, the affected entity) to its previous secure state. However, if the event occurs repeatedly or the time to recover is deemed too slow, or both, the system may remain in a high-stress, insecure state. In these situations, if the stress is to be reduced, the entity must be adapted to handle the event and put the system into a new, secure state. This paper applies research from a variety of disciplines to analyze the temporal effects of events on entities, and shows how resilience and adaptation contribute to the existence of energy security in energy systems. It underscores the importance of time when discussing the impact of events on an energy system and employs methods associated with reliability, notably mean time between failures (MTBF), mean time to recover (MTTR), and tolerance, to describe resilience and adaptation. The analysis is presented and discussed with examples using three common energy security indicators. (C) 2015 Elsevier Ltd. All rights reserved.