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When the WHO's Topic Advisory Group for Neurology (TAG) started work on revision of the ICD-10 diagnostic codes in June 2009, the issues were daunting. The existing classification was produced a generation ago and the need to move to the digital age was becoming imperative. Appreciating modern advances in genetics and immunology, and the consequent changes in understanding of the pathophysiology of disorders of the nervous system, WHO's charge to the TAG was to produce a comprehensive, up-to-date disease classification, while providing published or consensus evidence for each coding change. In addition, the task would be to focus on ways to reduce the treatment gap while considering the utility of ICD-11 when used in primary care and nonspecialist settings. The project mushroomed over the 3 years since our first meeting and continues to do so. The work was made even more difficult as the group needed to add "content models" for the major codes for the first time (i.e., providing a definition for each disorder, along with appropriate diagnostic tests and outcome). The ICD-11 is meant to be updated as new knowledge develops, rather than waiting some years for another whole-scale revision, but this process has yet to be defined.