Quality of mental health services provision for adults with intellectual disability and comorbid mental disorder is considered inadequate and therefore in need of further improvement. However, empirical findings on this topic are rather scarce. The MEMENTA-study therefore aims to compare mild and moderate intellectual disabled adults with a comorbid mental disorder to mild and moderate intellectual disabled adults who do not have a comorbid mental disorder with respect to clinical parameters. The MEMENTA-study is an observational, epidemiological, cross-sectional study. A representative sample of the target population is realised by the assessment of approximately n=600 intellectual disabled adults working at specialized facilities (sheltered workshops). A cluster sampling with probability proportional to institution and size (n of overall employed persons) will be followed by a simple random sampling of persons of these clustered facilities. A number of parameters such as severity of mental health impairment, needs for care, quality of life (QoL), caregiver burden, health services utilization, and costs for care will be assessed by using a set of well-established standardised instruments. Quality of mental health care will be assessed and examined by open questions to intellectual disabled adults with a comorbid mental disorder as well as informal caregivers and staff of caring institutions.