This document is intended for use by airspace planners as a basis for changing separation minima. Its primary objective is to guide airspace planners, ICAO Regional Offices and the regional planning groups and to assist them with implementation of CNS/ATM systems, particulary in relation to airspace planning and implementation of performance navigation. The methodology presented in this manual provides a framework by which airspace characteristics, aircraft capability and traffic demand can be assessed for the purpose of determining safe separation minima for en-route operations. The methodology has been designed to ensure that the intended safety level for a proposed airspace meets the required standard.