When a Public Accountant is contracted as an external auditor, he/she undertakes to issue a report in which he/she will express his/her opinion on the entity’s financial statements (it may be the case that the auditor should eventually refrain from giving an opinion, but this is an exception, as it is normal that when the services of an auditor are required it is expected to issue a report). By accepting to be appointed as an external auditor, the Public Counter also accepts the responsibilities of the Public Counter. They are inherent in their professional performance and are governed by the professional rules within which they must operate. Throughout this work, consideration will be given to the responsibilities of the auditor when acting in collaboration with other professionals. The specific cases to be examined are: (1) responsibility for the use of the work of another auditor; (2) consideration of internal audit work; and (3) use the work of an expert. To this end, reference will be made to the international auditing standards (ISAs) dealing with each specific topic, and some particular standards that exist from another body in this regard.