A system to identify strategies of justification used by teachers in the classroom was developed and used to classify secondary school mathematics teacher discourse. Justification of assertion and action were labeled "validation" and "vindication," respectively. Eight strategies were identified - six of validation and two of vindication. The most frequently occurring strategy of validation found was one in which a universal generalization is justified by means of a subsuming generalization. Other strategies identified included use of a counter-example and use of deductive proof. (Author/JG)