This document summarizes the key EU rules and principles governing public procurement contracts between contracting authorities. It notes that EU Treaties, Directives and national legislation provide the framework. The general obligation is that when a contracting authority needs goods, services or works, it must put the contract out to open competition according to EU Directives. Two exceptions established in EU case law are the Teckal case, which allows contracting without competition between entities under similar control, and the Hamburg Chamber case, which allows contracts solely between public authorities that do not involve private parties or affect future contracts. Subsequent case law has further clarified the exceptions. There are five criteria for allowed cooperation without competition between public bodies regarding public tasks. Some interesting questions are raised