Over the past 30 years the World Health Organization (WHO) has produced a number of assessment instruments intended for national and cross-cultural psychiatric research. WHO instruments have been tested and used in many collaborative studies involving more than 100 centres in different parts of the world. This article reviews the main WHO instruments for the assessment of (a) psychopathology, (b) disability, quality of life and satisfaction, (c) services, and (d) environment, and risks to mental health. The principles used in the development of WHO instruments, their translation and their use across cultures and settings are discussed.