Background and objectives
In most countries worldwide, general adult psychiatrists look after the mental health of adults with intellectual disabilities (ID) without appropriate specialist training in the field. It is, therefore, necessary to develop a practice guideline to help these clinicians to assess and diagnose psychiatric disorders in adults with ID.

Methods
We have developed an evidence and consensus-based practice guideline for the assessment and diagnosis of major functional psychiatric disorders in adults with ID. Several senior psychiatrists from different European countries formed a guideline development group who assessed the evidence gathered from a systematic literature search to produce the guideline.

Results
Adults with ID develop the same psychiatric disorders as their non-ID counterparts. The overall rate of major functional psychiatric disorders such as schizophrenia, depressive disorder, bipolar disorder, and anxiety disorders seems to be somewhere between 14.4-22.4%. However, if a wider definition of psychopathology and mental ill-health is considered including depressive and anxiety symptoms, behaviours of concern, and other neurodevelopmental disorders such as autism spectrum disorder the rate becomes much higher than that in the general population. The risk factors for psychiatric disorders seem similar in both the ID and the non-ID populations. However certain risk factors such as genetic burden, certain comorbidities, psychosocial adversities, and reduced coping capacity are more prevalent among adults with ID.

Conclusion
This guideline describes how the main symptoms of major psychiatric disorders may manifest differently in adults with ID and provides recommendations as to how to address these issues in day-to-day clinical practice.