Many militaries now have detention policies as a key strategic component of any military operation. They also assist other militaries and civilian arms of government in establishing rule of law and justice programmes which inevitably lead to detention operations. This feature of assisting third States to operate their detention programmes is one aspect of the duty to ensure respect for IHL that this chapter explores. This first part looks at the obligation to correct existing violations of IHL related to detention when assisting and partnering. The situations of ISAF support to detention operations in Afghanistan and of extraordinary rendition are discussed. The second part addresses the preventative responsibility to ensure respect, which is linked to States that are bystanders to an armed conflict and with no involvement in the activity of the offending State. Two situations, detention in Syria and in the US, are examined. The final part considers two approaches to developing a normative framework – Copenhagen Guidelines and the ICRC Strengthening IHL Project – to support future prevention of IHL violations and ensure respect for IHL by third States. The chapter concludes with a list of corrective and preventative measures States can adopt to ensure respect for IHL in relation to detention operations.