Search for responsibility in medicine became everyday. Anaesthetists are particularly exposed and will be, several times, confronted to it during their career. They have to have knowledge of some necessary elements to get to grips with expertise. Expertise can be asked by a penal jurisdiction. In that case, the anaesthetist can be directly and personally implicationed. When expertise is asked by a civil jurisdiction, it concerns anaesthetists, whichever the (liberal or employee of private). Expertise during administrative procedures concern hospital's anaesthetists. It is important to organize a preparatory meeting in any expertise. Praticians must collect together the complete medical file to establish the most exactly possible, chronology of facts. The anaesthetist can be accompanied by medical consultant appointed by the insurance companies and a lawyer. But he does not have to content with be represented by them. Presence in expertise is essential; praticians can so give evidence of their good faith and answer the expert's questions. Vagueness or doubt are never favorable to pratician. It is also, a responsible and respectful behavior toward the patient.