The author interprets disability on the basis of the "International Classification of Functioning, Disability and Health (ICF)". In this review the applied method is a purpose oriented, restricted survey of continuously increasing articles published during the last two decades. Definition and interpretation of secondary conditions are based on the cited international classification. It is noted that secondary conditions frequently develop during acute care and usually require rehabilitation. Significance of secondary conditions in rehabilitation is highlighted by selected examples of traumatic spinal cord injury, stroke, lower limb amputation and intellectual disabilities. The author states that there is an insufficiency of presently available knowledge and raises the need for the use of the International Classification of Functioning, Disability and Health as foundation of future research.