In France, there is a multitude of companies of all sizes and in all sectors. Given this heterogeneity of the French productive fabric, auditing assignments are governed by a set of laws and professional standards to enable auditors to express their opinion on the regularity, fairness and true and fair view of financial statements. Auditing is a regulated and independent profession that contributes to the quality and transparency of financial and accounting information issued by audited entities. Its missions are based on a legal obligation: it is a service of general interest. This dissertation will be devoted to external auditing, since it is the result of my internship assignment, carried out in a public accounting and auditing firm.