Mental health needs assessment has been encouraged by federal and state agencies for many years, and became a statutory requirement for some federally funded programs. There is still, however, no consensus on definitions of mental health need, methods for its assessment, or criteria to define successful needs assessment efforts. This review considers the separate components of needs assessment, together with techniques suitable for the measurement of each. Problems in defining the scope and boundaries of mental health need are reviewed, and then two general methods of assessment are described; epidemiologic surveys and social indicator analysis. Techniques, methodological issues, applications and limitations are discussed. Methods for assessment of current services which are needed to derive estimates of unmet need are reviewed. Finally, consideration is devoted to assessment of community desire for services, based on key informant interviews and surveys. In a concluding comment, the use of multiple data sources is recommended, selection of which should be determined by the goals of the particular study being conducted.