In everyday speech, we use the word fault to refer to responsibility for an undesired outcome. That’s generally how lawyers and insurance companies use it, too, but only when the facts of a situation meet specific criteria. Someone is legally at-fault for an accident or incident only if: They had a recognized duty not to put others in harm’s way; Their unreasonably dangerous decisions, actions, or failures to act violated that duty and caused a harmful accident or incident; and As a result, someone else suffered physical, emotional, or financial injuries. So, in the context of rear-end accidents, a person is at-fault if they owed others a duty not to act in a way or make a decision that risked causing a crash, they breached that obligation in a way that caused a collision, and someone else got hurt. By law, a person who meets those criteria will owe monetary damages (see below) to the rear-end accident’s injured victims.