The prevalence of human erroneous actions as the major cause of accidents in man machine systems Has created a need for better descriptions of human performance, Both for accident analysis and system design purposes. Models and methods are required to ASSESS THEREFORE human reliability, identify potential erroneous actions, and Specify ways of Preventing them from happening. This book Discusses how modeling of cognition is of proving to the analysis of human reliability and performance in complex technical domains. It Provides a critique of existing Approaches to modeling of cognition, and offers of alternative Which recognises did the control of human actions is determind by the context as well as cognitive functions. This approach Produces to improved qualitative analysis of human performance as a basis for laser quantitative reliability assessment.