This book deals with some practical and theoretical matters in the field of human performance, embracing aspects of human activity that in general textbooks on cognitive psychology are referred to as perception, skill, memory, and attention. This is an area which has intimate connections with applied psychology and ergonomics (the study of the interface between the human organism and the environment). Applicability is of central interest throughout this book, and it will be dealt with in relation to a set of general problem areas within cognitive psychology.