The first step in the consensus procedure was the performance of a systematic literature review. Details of this systematic review are described elsewhere [
28]. A wide range of definitions were in use, of which, four were cited by many other authors [
15,
18,
20,
26]. Pless and Douglas proposed a definition for children with any chronic physical disorder for use in epidemiological surveys [
20]. They described chronic illness as “a physical, usually non-fatal condition which lasted longer than 3 months in a given year, or necessitated a period of continuous hospitalisation of more than one month; of sufficient severity to interfere with the child’s ordinary activities in some degree.” Perrin et al. presented some recommendations for formulating a comprehensive, generic and flexible definition of chronic conditions in childhood [
18]. They recommended two levels: duration and impact. First, “a condition is considered chronic if it has lasted or is expected to last more than 3 months.” And a second specification is “to take into account the impact of the condition on the child. For example, the level of functional impairment or the use of medical attention greater than that expected for a child of the same age might be considered.” Stein et al. defined “children with ongoing health conditions” as children having disorders that: (1) have a biological, psychological or cognitive basis; (2) have lasted or are virtually certain to last for at least 1 year and (3) produce one or more sequelae, such as: (a) limitations of function, activities or social role in comparison with healthy peers in the general areas of physical, cognitive, emotional and social growth and development; (b) dependency of medication, special diet, medical technology, assertive devices or personal assistance to compensate for or minimise the limitations of function, activities or social role or (c) the need for medical care or related services, psychological services or educational services over and above the usual for the child’s age, or for special ongoing treatments, interventions or accommodation at home or in school [
26]. McPherson et al. defined a slightly different concept, namely “children with special health care needs” [
15]. They described these children as “those who have or are at increased risk for a chronic physical, developmental, behavioural, or emotional condition and who also require health and related services of a type or amount beyond that required by children generally.” Apart from these four often cited definitions, several different definitions were found, some of which were based on a list of diagnoses and others were based on the duration and consequences of the condition of the child, the so-called non-categorical definitions. However, the clarity and theoretical basis of these definitions varied considerably [
28].