This paper aims to understand the issues involved in introducing optimisation in complex computer based systems. It also proposes a framework within which optimisation may occur, and suggests the research that needs to be done. The paper follows through on software's dominance in complex systems to determine the feasibility of raising complex computer based system design to a new level of practice by enabling optimisation and refinement built upon architectural modelling. The paper draws on optimisation in chemical, control system and aeronautical engineering for its techniques and tools. The major issues are: how to apply optimisation to complex systems; how to apply refinement in a scalable, inclusive manner; how to extend architectural modelling to encompass optimisation and refinement. Additionally, a visual design guidance system will need to be developed for the designer, and an appropriate design process will need to be developed. The effect of a project built upon this framework could be to transform the way system/software architects design complex systems, and, have a profound effect on the effectiveness of all industries which deal with complex systems.