With the need to identify sustainable modes of food production for growing populations there has been a growing interest in the potential of organic farming. Although evidence suggests that organic systems can produce food in an environmentally efficient manner, the impacts of a widespread conversion to organic management are still uncertain. The research presented aimed to address this knowledge gap by completing a comprehensive and robust assessment of the food production, fossil energy-use and greenhouse gas impacts associated with a 100% conversion to organic farming in England and Wales. Firstly a structured literature review was carried out to determine the relative fossil-energy efficiency of organic systems. The sustainability of typical organic crop rotations was then assessed using a simulation model of crop-soil N dynamics. Land-use and production scenarios under 100% organic management were assessed through the development and application of a large-scale linear programming model that estimates levels of production under biophysical constraints, e.g. N supplies from biological fixation by legumes. A life-cycle assessment-based model was then applied to explore the extent to which a 100% conversion to organic farming could lead to improvements in greenhouse gas mitigation and fossil energy efficiency. The environmental assessment approach allowed for processes inside and outside of the immediate boundaries of the production systems to be assessed, with the question “what is affected by the change in levels of production?” asked throughout the process. The results revealed that whilst some organic systems offer improved performance in non-renewable resource use efficiency, a widespread conversion would result in a substantial decrease in domestic food production. Total food output expressed over five major food groups fell to 64% of a non-organic baseline. An increase in food imports would therefore be required to meet demand. From a greenhouse gas perspective, a 100% conversion to organic farming in England and Wales could lead to 6% decrease in the impacts of food production. The greenhouse gas mitigation potential of organic farming is strongly related to the use of clover and other legumes in place of manufactured N and lower concentrate feed rates in livestock production. Where the additional-land required under an organic scenario is newly cultivated, it is likely that any greenhouse gas benefit obtained would be offset. Total greenhouse gas emissions increased by an average of 28%, compared to a non-organic baseline, when the land use change impacts associated with increased food imports were included. When the soil carbon sequestration benefits obtained through organic farming are also included the net difference between the two production systems is lessened, however a fundamental question remains concerning the availability of overseas land (land use requirements under organic management increased by 29-47% depending on the scenario). Reducing the area of fertility-building ley within organic rotations is likely to improve productivities and reduce land-use requirements within organic farming systems. Improving crop cultivation practices, more effective cover-cropping and improved biological N-fixation could also help to improve N efficiency and productivity within organic systems. Changes to international organic standards in some areas may also improve the environmental sustainability of the sector, e.g. by allowing recycling of P from sewage treatment. Overall the research showed that whilst the adoption of organic farming can lead to improvements in environmental performance, a widespread conversion would need to be accompanied by substantial changes in diet and/or typical organic practices to become feasible from the perspectives of environmental impact and total food production.UK Engineering and Physical Sciences Research Council, The Ratcliff Foundation and The Organic Research Centre