A major goal in the development of Unmanned Aircraft Systems operations is access to non-segregated airspace. In order to achieve this, UAS must have a Sense & Avoid function analogous to the See & Avoid function of manned aircraft. In order to determine the ATM Collision Avoidance Requirements for UAS operating in non-segregated airspace EUROCONTROL have commissioned the CAUSE study, Phase 1 of which is reported here. The study uses results from previous EUROCONTROL safety studies of ACAS to demonstrate that there is a need for UAS to have a collision avoidance capability comparable to that delivered by ACAS on manned aircraft. The required performance capability is derived for a range of airspace regimes. The study also investigates to what extent carriage of ACAS by UAS might deliver this capability and the issues involved.