This is the book of a conference held at Leuven, Belgium from June 5-9 1979 under the same title. The conference was sponsored by the Scientific Affairs Division of the North Atlantic Treaty Organisation, Brussels. We would like to thank Dr. Bayraktar of NATO for his part in facilitating the organisation and support of the conference. We are also indebted to the authorities of the University of Leuven who provided excellent facilities and particularly to Professor Verhaegen of the Department of Psychology who acted as academic host to our conference. The aim of the conference was to bring together two groups of psychologists who have been developing in parallel their particular methods of studying and describing human behaviour. The skill psychologists began with the study of motor skills which are relatively easily observable in real jobs and recordable in the laboratory. More recently interests have shifted from motor skills through perceptual skills to the process skills where the operator is attending to many sources of information in the form of dials, charts and computer outputs and adjusting some process to maintain its stability and maximise the yield. Currently problems are arising of how to analyse situations in which several skilled individuals work closely together in small team performance. The social psychologists have followed an analogous but different path of progress.