The numerous terms used to describe the physical limitations of a subject to carry out the same level of work as before his illness have brought about an unfortunate confusion. The assessment of respiratory impairment is the responsibility of the doctor, whereas the decision as to whether the patient may resume his previous employment is often that of someone in a legal capacity. Many meetings during the last 20 years have been devoted to the assessment of respiratory impairment. A SEPCR working group recently proposed a classification in 3 categories based on simple functional examinations. The use of this grading system by chest physicians responsible for the assessment of the respiratory impairment will make comparison of data easier. Taking into account other elements of evaluation will give an opportunity of validation of this grading system. An international scale would do away with the disparities of the present evaluation. This scale based solely on simple tests would be one of the elements in the determination of respiratory impairment, and the clinical data must not be forgotten. The radiological aspects are to be treated carefully since they provide only indirect information about the respiratory function. Spirography is not sufficient and a study of respiratory exchanges is needed for an accurate evaluation. Examinations whose results depend neither on the collaboration of the subject nor the predicted values obtained according to the morphology are preferable. For this reason, the examination of blood gases is of utmost importance, despite the criticisms made by some specialists. The cardio-respiratory responses during exercise provide a very useful element for assessment. If the physician in charge does not have at his disposal the equipment for maximal exercise tests, he can ask for a short walking test on level ground. The response to this test gives relative approximations, but it can nevertheless be used as a useful complement to results recorded at rest.