While most researchers do agree now that situations may have an effect in the assessment of traits, the consequences have been neglected, so far: if situations affect the assessment of traits we have to take this fact into account in studies on reliability and validity of measurement instruments and their application. In the theoretical part of this article we provide a more formal exposition of this point, introducing the basic concepts of latent state–trait (LST) theory. LST theory and the associated models allow for the estimation of the situational impact on trait measures in non-experimental, correlational studies. In the empirical part, LST theory is applied to three well known trait questionnaires: the Freiburg Personality Inventory, the NEO Five-Factor Inventory and the Eysenck Personality Inventory. It is shown that significant proportions of the variances of the scales of these questionnaires are due to situational effects. The following consequences of this finding are discussed, (i) Instead of the reliability coefficient, the proportion of variance due to the latent trait, the consistency coefficient, should be used for the estimation of confidence intervals for trait scores, (ii) To reduce the situational effects on trait estimates it may be useful to base such an estimate on several occasions, i.e., to aggregate data across occasions. (iii) Reliability and validity studies should not only be based on a sample of persons representative of those to whom the test will be applied; they should also be conducted in situational contexts representative of the intended applications.