The World Health Organization (WHO) Department of Mental Health and Substance Abuse has developed a systematic program of field studies to evaluate and improve the clinical utility of the proposed diagnostic guidelines for mental and behavioral disorders in the Eleventh Revision of the International Classification of Diseases and Related Health Problems (ICD-11). The clinical utility of a diagnostic classification is critical to its function as the interface between health encounters and health information, and to making the ICD-11 be a more effective tool for helping the WHO's 194 member countries, including the United States, reduce the global disease burden of mental disorders. This article describes the WHO's efforts to develop a science of clinical utility in regard to one of the two major classification systems for mental disorders. We present the rationale and methodologies for an integrated and complementary set of field study strategies, including large international surveys, formative field studies of the structure of clinicians' conceptualization of mental disorders, case-controlled field studies using experimental methodologies to evaluate the impact of proposed changes to the diagnostic guidelines on clinicians' diagnostic decision making, and ecological implementation field studies of clinical utility in the global settings in which the guidelines will ultimately be implemented. The results of these studies have already been used in making decisions about the structure and content of ICD-11. If clinical utility is indeed among the highest aims of diagnostic systems for mental disorders, as their developers routinely claim, future revision efforts should continue to build on these efforts. (PsycINFO Database Record