When the function of a service, support, or assistance animal relates to a mental or emotional disability, a psychologist may be asked to write a letter for a patient seeking to live with the animal, usually a dog, or bring it onto an airplane or take it into a restaurant. Understanding the function of the dog will be important for the psychologist as there is no one-size-fits-all letter for all specialized dogs. This article analyzes the law and describes letters that helped patients and others that harmed them. The authors make recommendations about how to write such letters. Abstract published by arrangement with Taylor and Francis.