The heart of the KMOS instrument is a complex optical system with over 300 separate optical paths. The optical design is spread between 4 sub-systems which have been designed at three different institutions. In order that the end to end performance of the final design can be monitored and controlled it is necessary to specify the performance and interface requirements of each sub-system clearly. This paper describes the parameters that were necessary to control so that the sub-system designs could be carried out independently while maintaining visibility and control of the end to end performance. The method of apportioning the budgets between the sub-systems and the modeling performed to verify compliance is also described.© (2008) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.