The approval of the International Classification of Functioning, Disability and Health (ICF) [1] by the World Health Assembly in May 2001 has marked a paradigm shift in the way health and disability are understood and measured. Until recently, “health” has only been seen as the opposite of death or disease. Traditional health indicators have mostly focused on mortality and morbidity. On the other hand, “disability” has been seen as an unrelated entity, either as a medical issue of bodily impairments such as blindness and deafness or as an imposed restriction on the individual that hinders him/her from taking part in daily life activities. ICF has brought these concepts into a comprehensive whole of multiple dimensions of human functioning synthesizing biological, psychological, social and environmental aspects. ICF, thus, presents health and disability in a single spectrum. Traditionally these areas have been thought separately and at times put into polarization. However, a detailed analysis of the domains that make up health and disability shows that these two basic constructs are in fact different manifestations of the same domains of functioning such as seeing, hearing and many others.

Formulating human functioning in multiple dimensions under a biopsychosocial view is not new to a number of medical fields, such as rehabilitative medicine, mental health, physical, occupational and speech and language therapy, and in nursing home and home care settings. What is new is that with the adoption of ICF we have a globally agreed-on conceptual framework and common language to document and code functional status information.

The Body component comprises two classifications, one for the functions of the body systems, and one for the body structures. The chapters in both classifications are organized according to the body systems.

A list of Environmental Factors is the first component of Contextual Factors. Environmental factors have an impact on all components of functioning and disability and are organized in sequence from the individual’s most immediate environment to the general environment.

Personal Factors is also a component of Contextual Factors but they are not classified in ICF because of the large social and cultural variance associated with them.

The components of Functioning and Disability in Part 1 of ICF can be expressed in two ways. On the one hand, they can be used to indicate problems (e.g. impairment, activity limitation or participation restriction summarized under the umbrella term disability), on the other hand they can indicate non-problematic (i.e. neutral) aspects of health and health-related states summarized under the umbrella term functioning.

These components of functioning and disability are interpreted by means of four separate but related constructs. These constructs are operationalized by using qualifiers. Body functions and structures can be interpreted by means of changes in physiological systems or in anatomical structures. For the Activities and Participation component, two constructs are available: capacity and performance.

As a "second generation" classification, ICF is concept driven. Apart from classifying the universe of disability, ICF also provides a conceptual framework for understanding disability.

At the core of the ICF concept of health and disability is the notion that disability is a multidimensional and universal phenomena placed on a continuum with health. Human functioning is understood as a continuum of health states and every human being exhibits one or another degree of functioning in each domain, at the body, person and society levels.

ICF conceptualises disability not solely as a problem that resides in the individual, but as a health experience that occurs in a context. Disability and functioning are, according to the ICF model, outcomes of interactions between health conditions (diseases, disorders and injuries) and contextual factors. The bio-psychosocial model embedded in the ICF broadens the perspective of disability and allows medical, individual, social, and environmental influences on functioning and disability to be examined.

Moreover, ICF is grounded in the principle of universality, namely that functioning and disability are applicable to all people, irrespective of health condition, and in particular that disability – or decrement in functioning at one or more levels – is not the mark of a specific minority class of people, but is a feature of the human condition, which is, epidemiologically speaking, over the life-span, a universal phenomena. In addition, ICF is committed to the principle of parity, which states that the functional status is not determined by background etiology, and in particular by whether one has a ‘physical’ rather than ’mental’ health condition.

Figure 1 depicts the components of the ICF in an interactive model. The health condition (a disease or disorder) may impact functioning at 3 mutually interacting levels: in relation to the body, at the level of activities, and at the level of participation in society. The way health condition impacts functioning should also be considered within the context of environmental and personal factors.

The ICF model and its underlying principles represent a significant development from its predecessor, the International Classification of Impairment, Disability and Handicap (ICIDH) [2]. In the ICIDH, disability was understood as a limitation in the person’s activity that resulted from impairment. Neither disabilities nor handicaps could be assessed in terms of degree of severity. Environmental factors were acknowledged but not classified and no linkages between disability and health status measurement were made. Due to these limitations, ICIDH was generally viewed as flawed and so was ignored by disability data users in general and by advocates of the social model of disability (especially organizations representing persons with disabilities) in particular. In response to these and other criticisms, the ICF was developed over a seven-year period in an international collaborative process and validated by means of field trails in over 70 countries before officially being endorsed by all WHO Member States in 2001.

ICF was conceived as a common language and data standard, capable of being used for multiple purposes and in different settings. Since its adoption by the World Health Assembly in 2001, the ICF has been implemented in a variety of ways at various levels. The listing of examples below provides an overview of where and how the classification is used.

Applications at population level:

Applications at health and social service level:

Using ICF in specialty settings may require to focus on a particular range and granularity of ICF codes. For example, a clinician dealing with patients with depression will need a wider range of categories to identify the area of mental functions and interpersonal interactions and relationships. A speech and language therapist, on the other hand, will require detailed description of voice and speech functions and related structures.

In response to this information need in specialized clinical settings, ICF core sets [30] have been developed. Using the ICF Checklist as a starting point, the ICF Core Sets identify the most relevant ICF domains for a series of health conditions. For clinicians who choose to continue to use their existing clinical measures and map them to the ICF framework, semantic maps have been developed alongside with mapping rules and procedures.

Sensitization and training of stakeholders from health, social service and educational institutions proved to be a critical element in promoting the ICF implementation in these sectors. Over the last years, a number of ICF training tools and awareness raising and education activities on ICF were conducted [31, 32]. Currently a web-based ICF e-learning tool is being developed and field tested. The introduction module of the tool is expected to be launched by the end of 2010.

As a classification the ICF is meant to be a "living document" and therefore has to be kept up-to-date and developed further.

To update the ICF, WHO has established a web-based ICF update platform, which is open to any person in the general public. The platform also provides a workbench for experts of the WHO Functioning and Disability Reference Group (FDRG) and the Update and Revision Committee (URC) to process update proposals. The application collects update proposals in a structured and organized manner. This is done by asking the user to fill a form in which the user explains the proposal as well as the rationale behind the proposal. In addition to this, he/she can provide links to web pages such as publication references from PubMed or can upload documents that are relevant to the proposal. Subsequently, each proposal is reviewed online by experts from the FDRG in terms of its compliance with established criteria like: validity, reliability, added value as classification entity; consistency with ICF structure, concept and taxonomic principles; rationale and evidence base. Following the review, experts in the URC decide whether the proposal should be implemented or rejected.

An important milestone in the further development of the ICF was the publication of the ICF Children and Youth version (ICF-CY) [33] in 2007. As the first ICF-derived classification, the ICF-CY is expanding the coverage of the main ICF volume by providing specific content and additional detail to more fully cover the body functions and structures, activities and participation, and environments of particular relevance to infants, toddlers, children and adolescents. An important novelty introduced by the ICF-CY is that it captures and operationalizes the notion of the “developing child” and takes into account the following: (a) the child needs to be viewed in the context of the family; (b) the nature and forms of participation change dramatically from dependent relationships in infancy to complex life situations in adolescence; (c) mirroring developmental changes in participation, the nature and number of environments change as well; and (d) lags in emergence of functions or acquisition of skills may reflect developmental delay rather than impairments or stable limitations.

One of the most important prospects for the future development and implementation of the ICF emerge from aligning ICF and International Classification of Diseases (ICD) [34] in the context of the ongoing ICD revision process.

ICF puts the notions of ‘health’ and ‘disability’ in a new light. It acknowledges that every human being can experience a decrement in health and thereby experience some disability. This is not something that happens to only a minority of humanity. ICF thus ‘mainstreams’ the experience of disability and recognizes it as a universal human experience. By shifting the focus from cause to impact it places all health conditions on an equal footing allowing them to be compared using a common metric – the ruler of health and disability.

Since its publication in 2001, the ICF has been gradually implemented in a variety of settings and sectors. At clinical level, its uptake has been most noticeable in the area of medical, social and occupational rehabilitation. The ICF information on functioning and disability enriches the diagnostic information in the ICD, providing a broader, more meaningful picture of the patient’s health, which can be used for better management decisions.