Performance is evaluated by a set of criteria selected by the company and recorded in a document drawn up by the "management controllers" and intended for managers who can analyze the differences between objectives and results in order to decide on corrective actions. Among the elements that make up the internal control environment is the internal audit function. Internal auditing consists of observing, studying and analyzing the situation of behaviors, facts, structures and social documents in relation to the company's internal reference systems. This allows us to identify discrepancies, to search for causes, to study their consequences or implications to identify the company's risks in order to better control them, to eliminate them and to better ensure the survival of internal control procedures.The theme treated in this thesis will allow us to better understand the concept of internal audit, its role within the company, its objectives and the methodology accepted for the conduct of an audit mission.