From the 1939-45 war onwards hill-farming was increasingly debated about and legislated for as a discrete sector of agriculture. This study, in taking the three counties of Wales with the highest proportion of land over 500' - thus providing the most 'pure' exmples of this type of farming - seeks to explore how a 'hill-farm problem' emerged and was defined. This is accomplished through a close examination of the interrelationships between scientific/technological innovations and ecological, social, and economic patterns. Use is made of contemporary data collected primarily by agricultural economists but also by grassland scientists and other agrarian researchers, supplemented by records of farmers' bodies, official bureaucracies and involved individuals. This writer's findings suggest that contrary to the opinion of some authorities, the farmers have been highly responsive to technological and market changes, the consequences being a shift towards a high degree of specialization, entailing decreasing levels of manpower and rural depopulation. The pattern that emerged was predicated on substantial state subsidies, encouraging production rather than a solution to the hill problem, and productive of an in-built fragility in the system of farming, now clearly exposed by the wider crisis in European Common Agricultural Policy. The way out of these difficulties may entail a radical reappraisal of agricultural and food production going beyond the current political discussion on quotas, alternative farming systems, set-aside policy and conservation. An approach favoured by the writer would involve recognition of the entropy factor with its stress on the long-view of eco-systems, a theoretical approach not incompatible with the views of Sir George Stapledon, one of the central figures in the developments portrayed in this study.