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The advent of novel techniques for tracing connections in vivo, such as diffusion tensor magnetic resonance imaging tractography, allows us, for the first time in the human brain, to study the microstructural integrity of white matter fibres and perform virtual dissections of large scale neurocognitive networks. This review will outline the advantages and limitations of applying diffusion tensor magnetic resonance imaging to the study of cognitive and behavioural disorders in neurology and psychiatry.Diffusion tensor magnetic resonance imaging has been used to re-explore the anatomy of white matter tracts in the living human brain and to create connectional models of brain function. Beyond its application to classical disconnection syndromes, diffusion tensor magnetic resonance imaging is becoming an important tool to extend the disconnectionist paradigm to neurodevelopmental and neurodegenerative disorders.For the first time, we are able to correlate disconnecting lesions with clinical symptoms in vivo and test the disconnection mechanism directly in cognitive disorders. With diffusion tensor magnetic resonance imaging tractography alone and in combination with other magnetic resonance imaging techniques, researchers are able to detect abnormalities in white matter that are not visible with conventional magnetic resonance imaging.