A fault tree is a diagrammatic representation of the relationship between the lower-level events that may represent hardware failures, software failures, human error, etc., and a system-level event. The fault tree depicts the propagation of the lower-level events that cause a system-level undesired or top event. It is made up of successive levels such that each event is generated from lower levels via various logic operators (gates). The lowest-level events in each branch of the tree are generally referred to as primary events or basic events, but they may also be referred to as terminal events.

The primary events of a fault tree are those events that, for one reason or another, have not been further developed. Probabilities for these events must be provided if the fault tree is to be used for computing the probability of the top event. There are four types of primary events:

In addition, a Spare event is included in the literature to model spare components. For additional information, refer to Events and Gates.