The ICD-11 chapter on mental disorders is going to be approved by the World Health Assembly in 2018. An effort has been made to harmonize the two main diagnostic systems existing in psychiatry (the ICD and the DSM), and indeed the metastructure of the two systems will be the same. There will be, however, several differences between the two systems. There will be some diagnostic categories in the ICD-11, which do not appear in the DSM-5, such as complex post-traumatic stress disorder and prolonged grief disorder, and there will be some diagnostic cate-gories that appear in the DSM-5 but will not be included in the ICD-11, such as disruptive mood dysregulation disor-der. Conditions related to sexual health and sleep-wake disorders will appear in chapters of the ICD-11 diagnostic system different from that on mental disorders. The new diagnostic system has been tested through four sets of field studies: international surveys of views of psychiatrists and psychologists on the classification of mental disor-ders; formative field studies (aimed to guide decisions on the basic structure of the classification), Internet-based field studies (using vignette methodology to examine decision making in relation to ICD-11 categories), and clinic-based field studies (assessing the reliability and clinical utility of the diagnostic guidelines with real patients). [Anadolu Psikiyatri Derg 2018; 19(4.000): 339-345]