With reference to Kierkegaard's religious writings and other pseudonymous works, Kierkegaard's concept of anxiety is explored with a primary focus on the book The Concept of Anxiety. Anxiety itself is shown to be the eminent term for a moment in the human being's psychological development that Kierkegaard also discusses in other terms. In particular, it is the mode in which human being's apprehension of their possible freedom first appears in consciousness and, as such, is not to be confused with pathological anxiety, melancholy, or other negative states. At the same time, Kierkegaard regards human beings as typically failing to realize the possibility of freedom and when this happens anxiety becomes the trigger for despair or sin. The Element concludes by considering the influence of Kierkegaard on discussions of anxiety in Heidegger, Sartre, and Tillich and the relevance of his analysis to the contemporary crisis of social anxiety.