(1) A method for representing the logical combinations of various system states which lead to a particular outcome (Top event). With suitable data it can be used to quantify the probability or frequency of an event. (2) FTA is a technique, which can be either qualitative or quantitative, by which conditions and factors that can contribute to a specified undesired event (called the top event) are deductively identified, organised in a logical manner and represented pictorially. The faults identified in the tree can be events that are associated with the component hardware failures, human errors or any other pertinent events which lead to the undesired event. Starting with the top event, the possible causes or fault modes of the next lower functional system level are identified. Following stepwise identification of undesirable system operation to successively lower system levels will lead to the desired system level, which is usually the component fault mode. FTA affords a disciplined approach which is highly systematic, but at the same time sufficiently flexible to allow analysis of a variety of factors, including human interactions and physical phenomena. The application of the "top-down" approach, implicit in the technique, focuses attention on those effects of failure which are directly related to the top event. This is a distinct advantage, although is may also lead to missing effects which are important elsewhere. FTA is especially useful for analysing systems with many interfaces and interactions. The pictorial representation leads to an easy understanding of the system behaviour and the factors included, but as the trees are often large, processing of fault trees may require computer systems. This feature also makes the verification of the fault tree difficult. FTA may be used for hazard identification, although it is primarily used in risk assessment as a tool to provide an estimate of failure probabilities or frequencies