Recognizing the diagnostic challenges that clinicians face when attempting to arrive at an accurate psychiatric diagnosis for individuals with intellectual and developmental disabilities (IDD) co-occurring with mental illness (MI), in 2007 the National Association for the Dually Diagnosed (NADD), in association with the American Psychiatric Association (APA), published Diagnostic Manual-Intellectual Disability ("DM-ID"): A Textbook of Diagnosis of Mental Disorders in Persons with Intellectual Disability (Fletcher, Loschen, Stavrakaki, & First, 2007). The "DM-ID" was designed as a companion to the "DSM-IV-TR" and aimed to assist clinicians to arrive at a more accurate "DSM-IV-TR" diagnosis for individuals with IDD. In 2013, the American Psychiatric Association published the "DSM-5," thus necessitating revision of the "DM-ID" to incorporate the changes from the "DSM-IV-TR" to the "DSM-5." The authors discuss the need for and development of the original "DM-ID" and changes in the "DSM-5." The authors then offer insight into several chapters in the DM-ID-2 across the lifespan of individuals with IDD, looking at the changes in the "DSM-5" and how these impact the ascertainment of mental disorders in individuals with IDD.