This booklet is intended for general practitioners who regularly or occasionally perform medical assessments, for specialists in rheumatology or orthopedic surgery who have to give their expert opinion in their practice, and for occupational physicians who have to assess the fitness of an employee. Indeed, the paying organizations (private or public), the administrative supervisors, often question them on the legitimacy of a work stoppage or prolonged care, on the compensation of a post-traumatic injury or following an accident at work or in the service, on the validity of a request for occupational disease... In the case of civil servants or local authority employees, the question is whether the condition entitles the employee to ordinary or long-term sick leave or even long-term leave. Sometimes, the expert has to arbitrate a dispute between the doctor in charge of the health insurance fund and the patient's attending doctor. In these situations, the dialogue between the expert physician and the patient is not the same as in medical care.