Task analysis is a fundamental methodology in the assessment and reduction of human error. A wide variety of different task analysis methods exist, and it would be impracticable to describe all these techniques here. Instead, the intention is to describe representative methodologies applicable to different types of task. The term Task Analysis (TA) can be applied very broadly to encompass a wide variety of human factors techniques. Nearly all task analysis techniques provide, as a minimum, a description of the observable aspects of operator behavior at various levels of detail, together with some indications of the structure of the task. These will be referred to as action oriented approaches. Other techniques focus on the mental processes which underlie observable behavior, e.g. decision making and problem solving. These are referred to as cognitive approaches. TA methods can be used to eliminate the preconditions that give rise to errors before they occur. They can be used as an aid in the design stage of a new system, or the modification of an existing system. They can also be used as part of an audit of an existing system.