Behavioural assessment is a psychological assessment paradigm that emphasizes empirically supported, multimethod and multi-informant assessment of specific, observable behaviours and contemporaneous causal variables in the natural environment. The behavioural assessment paradigm stresses the use of well-validated assessment instruments and assumptions that social/environmental, cognitive, and physiological variables are often important sources of behaviour variance. The behavioural assessment paradigm has had a major influence on the field of psychological assessment. It has affected the way research on the causes of behaviour disorders is conducted, the way treatment processes and outcomes are evaluated, and the way treatment decisions are made. The goal of this work is to present the characteristics and underlying assumptions of the behavioural assessment paradigm and to show how they affect the strategies of behavioural assessment. Although all of the concepts and strategies discussed in this volume are applicable in research, the author focuses on the use of behavioural assessment to guide clinical judgements.