Recent work has demonstrated that incidents, accidents and disasters tend to result from complex socio-technical failures, rather than just 'human error' on the one hand, or simple technical failures on the other. For the reduction of accidents, therefore, it is necessary to deal with systems factors, in which both technical and human-factors elements play an equal and complementary role. However, many of the existing techniques in ergonomics and risk management concentrate on plant/technical issues and downplay human factors and 'subjectivity'. The present text describes a body of theory and data which addresses this issue squarely, drawing on systems theory and applied psychology, and which stresses the importance of human agency within systems. The central roles of social consensus and reliability, and the nature of verbal reports and 'functional discourse' are explained in some detail. This book therefore presents a new, 'Qualitative Systems Approach' to safety management, offering both greater safety and economic savings. It presents a series of methodological 'tools' whose reliability and validity have been shown through extensive work in the rail and nuclear industries and which allow organisational and systems failures to be analysed much more effectively in terms of quantity, precision and usefulness. This is a textbook for undergraduate and graduate students in occupational psychology, human factors, ergonomics and HCI, and the sociology of disasters and risk. It is also useful for safety managers and professionals in many safety critical firms and organisations, reliability engineers, risk managers, and human factors specialists.