Although the primary focus of this text is not to provide a reference on mental illness conditions (see preface), this chapter is included for several reasons. First, anxiety disorders are the most common psychosocial diagnoses worldwide, making it a virtual certainty that anyone in the education profession will work with affected students during their career. Second, this chapter provides a foundation that will facilitate appreciation of the content in the succeeding chapter. Third, much of this material links to other chapters herein, and hopefully enhances the understanding that the information throughout the text is intimately and irrevocably interrelated. The psychosocial conditions described can impact individuals along a continuum ranging from impairing to life threatening. Following this discussion is a chapter discussing the intricacies-and necessity-of intervening in the school setting with an anxiety disorder largely unknown, yet not rare.