Background and Objectives: This paper analyses the different aspects related to the conceptualization and assessment of Intellectual Developmental Disorders/Intellectual Disability (IDD/ID) following the Person-centered Integrative Diagnostic (PID) model of the International Network for Person-centered Medicine, with a main emphasis on the health status and health self-perception. Methods: Conceptual paper, including expert opinions based on literature review. Results: The conceptualization of IDD/ID should shift the traditional over-reliance on the intelligence (IQ) score in favour of the daily life expression of specific cognitive functions and the determination of the levels of severity of intellectual functioning, that is currently based on the person’s IQ score, should be reached through a system that is predicated on the person’s satisfaction attainment towards life. The assessment of cognition should be aimed at identifying those dysfunctions that have the highest impact on individual behaviour, skills, adaptation, autonomy, and quality of life across the life span, highlighting personal cognitive strengths and weaknesses that can be worthwhile for the planning of effective interventions. Conclusions: Authors conclude that the application of the PID model to IDD/ID represents a prototypical example of how this approach can be useful for understanding complex constructs in health care. An overview of the main factors related to the implementation of the person-centered care model by health systems and services is also provided.