FTA is a method for analyzing causes of hazards. FTAs use Boolean logic (gates) to describe combinations of individual faults than can create a hazardous event. Each level of the tree lists the lower level events that are necessary to cause the event shown in the level above it.

The tree is written as a Boolean expression to show the specific combination of identified basic events sufficient to cause the undesired top level event. If the individual probabilities for all the basic events are known (not feasible in most abstract cases), the frequency of the top event can be calculated.

Often the most difficult part of creating a fault tree is the determination of the top level event. The selection of the top event is crucial since hazards in the system will not be comprehensive unless the fault trees are drawn for all significant top level events.

Once the top event has been defined, the next step is to determine the events related to the top event and the logical relations between them, using logic symbols to define the relations. The most frequently used symbols for fault trees are AND and OR gates. The output of an AND gate only exists if all the the input events exist (it represents combinations of events). The output of an OR gate exists provided at least one on the input events exist (it shows single events that can cause the output event). NAND and NOR gates are used primarily to define countermeasures that, if true, will allow the system to tolerate conditions that would otherwise result in safety hazards or machine failure.