The community version of the MRC Needs for Care Assessment Schedule (NFCAS-C) is a new instrument designed for the psychiatric conditions seen in general populations. Its principles are based on the original Needs for Care Assessment, although that was developed for the very different population of those with long-standing mental illness (Brewin et al. 1987). The NFCAS-C is designed to reproduce, in an itemized and systematic manner, the functioning of well-organized primary care and psychiatric services. It is primarily intended for research purposes, in particular, the comparison of treatment needs and services in different populations. In this paper we describe the problems of assessing the need for psychiatric care in general populations and the rationale and development of the instrument, while in its companion we present the results of a reliability study (Lesage et al. 1995).