This paper examines the opportunities and challenges in co-operation between internal and external auditors in the public sector and provides examples of good practices from Supreme Audit Institutions and government internal audit bodies. It identifies public auditors’ perception of benefits and risks of co-operation between internal and external auditors, supported by survey results from the OECD Auditors Alliance members. Good practices were collected by survey and desk research and included examples from Austria, Brazil, Canada, Finland, Japan, Korea, Lithuania, the United Kingdom, and the United States. Additionally, the paper suggests insight to enhance the co-operation in terms of legal framework, communication, mutual trust and feedback between internal and external auditors, and central harmonisation function for the internal control and internal audit in the government.