Dual diagnosis is often referred to those with substance abuse disorders and mental illness. But, when someone with intellectual disability with co-occurring mental illness and behavioral issues, the term ‘dual diagnosis” is also used. When individuals with intellectual disability exhibit challenging behaviors, it will be difficult for the professionals to make a clear assessment of what the underlying causes of those behaviors are due to lack of coping skills, limited verbal skills, cognitive limitations, or even medical problems leading to pain and discomfort for aggression of these individuals. The result is poor diagnostic gaps in service for such individuals. When treating people with mental illness, a general tendency is to initially attribute any physical symptoms to their psychiatric illness. But, what about some individuals with intellectual disability who are unable to describe their physical distress, and even those who are able to speak but certainly not reliable in their self-reporting of the problem? If clinicians lack experience in developmental disability, individuals can be victims of poor diagnosis. There is an increased prevalence of mental illness and dementia in people with intellectual disability compared to population at large. There is significant gaps in mental health service for those with intellectual disability. There is shortage of psychiatrists specialized in intellectual disability that leads to other untrained professionals acting as liaison between individuals, family and caregivers and the psychiatrist. There is a growing need for new strategies to work with individuals with dual diagnosis, their caregivers and treatment team. Simple interventions with quality life improvements to help families and dually diagnosed individuals will tremendously help. there are significant gaps in services to dually diagnosed individuals that require crisis response, acute care, and social supports. Above all, the systems of mental health and developmental disability need to interact and communicate better for an integrated treatment plan for dually diagnosed individuals.