This chapter introduces the concepts of internalizing disorders and anxiety. The development of normal fears, worries, and anxieties are discussed. The main focus of the chapter is an examination of the following disorders: specific phobias, social anxiety disorder (social phobia), selective mutism, separation anxiety disorder, school refusal, generalized anxiety disorder, panic attacks and panic disorder, and obsessive-compulsive disorder. Through an examination of key concepts, issues, and research and enriched by case material, tables, and illustrations, the definition/diagnostic criteria, description, epidemiology, patterns of co-occurrence, developmental course, and etiology of the various disorders are discussed. Current knowledge regarding the assessment, treatment, and prevention of these disorders is also presented. In addition, the role of culture and ethnicity in the development, presentation, assessment, and treatment of these disorders is highlighted.