The aim of the paper is to show that research on temperament is inescapably bound with the concept of trait as applied in personality research. It is the individual differences approach on which temperament studies are based, and traits are the basic units by means of which these differences are described. Taking as a point of departure the definition of trait understood as a relatively stable and individual-specific generalized tendency to behave or react in a certain way expressed in a variety of situations, the hypothetical status of temperament traits is discussed. Special attention is paid to states and behaviour by means of which temperament traits are inferred as well as to the biological and environmental determinants of these traits. Temperamental traits constitute only a part of the personality structure viewed from the perspective of individual differences and this perspective is only one of the many from which the complex nature of personality should be viewed. Copyright © 2001 John Wiley & Sons, Ltd.