Halberg, N. 2012. Assessment of the environmental sustainability of organic farming: Definitions, indicators and the major challenges. Can. J. Plant Sci. 92: 981-996. The debate over agricultural sustainability continues due to the challenges of reducing externalities of intensive farming methods and preserving vital natural capital, but many definitions of sustainability are too wide to allow for a prioritized assessment. This paper uses a more narrow definition of agricultural sustainability focusing on the functional integrity of a system to highlight specific aspects of vital importance for the long-term resilience and reproducibility of agricultural systems. Key areas of resource sufficiency are also identified. Based on a review of scientific literature the relative sustainability of organic agriculture is assessed with a focus on environmental impact and resource use in Europe and North America. While there are many examples of organic agriculture with improved performance in terms of soil fertility and preservation of biodiversity, in other aspects - such as resource use per kilogram product - the difference to conventional farming is less important. The paper presents a framework for selection of indicators based on the principles of organic agriculture which may be used to monitor and improve the performance of organic agriculture with respect to functional integrity and resource sufficiency. The differences between comparable organic farms may be used for improving farm practices through a benchmarking process.