After the drought which struck the countries in the Sahelian area from 1971 to 1973 FAO and the World Food Programme underlook a systematic monitoring of rainfall during the growing season in that area in order to detect at an early date any anomalies in the quantity and distribution of rainfall which might harmfully affect crops. In 1976 the simple rainfall monitoring based on departures from "normal" rainfall evolved into an assessment of the cumulative water balance of crops and a short note was printed and widely distributed (Frere and Popov, 1976). As a result of this note the Plant Production and Protection Division received many requests from governments for the introduction at national level of method designed by the Crop Ecology and Genetic Resources Unit (AGPE). Consequently specific mission went to Ethiopia, Algeria, Tanzania, Ghana, Togo, Nigeria and Haiti and many other countries, such as Bangladesh, Indonesia, Venezuela and Sudan, have demonstrated interest in this agrometeorological crop monitoring and forecasting. The purpose of this publication is to present the method as it now stands, together with some results obtained in a wide range of environments. The experience gained so far already allows a good assessment to be made of the usefulness of the methodd in countries where water availability is the main constraint for crop production. It is also clear that in many instance the application of the method will foster the collection of sets of good biological data (yield and phenology) which, when interpreted together with the agroclimatological data, will permit the progressive establishment of fully quantitative models. The main advantage of the present method, however, is that the work can be initiated immediately without sophisticated means, even if detailed quantitative yield data are not readily available