This short review begins by defining some basic medico-legal concepts such as "impairment" and "disability" and gives the corresponding terms in French and German. It is then shown how, in stable obstructive and restrictive respiratory diseases such as COPD and lung fibrosis, the degree of impairment can be assessed on the basis of FEV1 and indices of gas exchange. In the case of bronchial asthma, however, with its typically variable degree of airflow limitation, the amount of reversibility and treatment necessary to achieve optimum bronchodilatation must be taken into account. This can best be done using a score system. Impairment represents a base but in no way equals the final percentage of disability pension or compensation, which are always assessed by the competent administrative authority. However, it is the physician who specifies the amount and type of work an individual patient, with his or her particular degree of disability, can or cannot be expected to do.