ISO 19011

ISO 19011 is an international standard that sets forth guidelines for:
quality management systems auditing
environmental management systems auditing
It is developed by the International Organization for Standardization.
The standard offer four resources to organizations to "save time, effort and money":
A clear explanation of the principles of management systems auditing.
Guidance on the management of audit programmes.
Guidance on the conduct of internal or external audits.
Advice on the competence and evaluation of auditors.