The concept of anxiety has been used in Behavior Analysis under the control of different events or relations. In this article, we offer a theoretical and conceptual review of behavior-analytic approaches for anxiety, and of the behavioral relations that are highlighted in the literature. We start with a description of the current usages of the concept of anxiety. We point out that such usages vary from the role assigned to physiological changes, to the definition of respondent and operant relations (verbal and non-verbal) and to the implications for verbal therapy. We, then, discuss such variations as complementary explanations for a complex phenomenon, in which events acquire several functions, through processes of direct and indirect cond...