This paper sets out to consider O.R. as a problem-solving methodology in relation to other systems-based problem-solving methodologies. A ‘system of systems methodologies’ is developed as the interrelationship between different methodologies is examined along with their relative efficacy in solving problems in various real-world problem contexts. In a final section the conclusions and benefits which stem from the analysis are presented. The analysis points to the need for a co-ordinated research programme designed to deepen understanding of different problem contexts and the type of problem-solving methodology appropriate to each.