ln many cases, the failure of agricultural development projects has been attributed to a lack of understanding of farmers varied circumstances, and of the practices and strategies they adopt Lo achieve their goals. In order to overcome this problem, the need for a preliminary phase of a directed diagnosis prior to any research and development actions, is now widely recognized. Its general objectives are to understand the internal coherence and to classify the existing farming systems according to their socio - economic objectives and the kinds of problems faced in reaching them. This paper presents a set of tools articulated in to a global, systemic and historical approach based on the coherence concept of agrarian system. After a definition of its main interrelated variables, each step of the methodology of diagnosis proposed here is described and illustrated by examples from the rice - growing area of Sathing Phra. The first stage involves an agro - ecological zonation and analysis of the recent technical and socio - economic transformations of the regional context in order to understand the organization of the landscape and its evolution. This is followed by the selection of different types of villages and a limited number of agricultural production systems for an in - depth analysis of their functioning pattern. The subsequent building of a functional typology of the local farming systems based on the differentiation of their socioeconomic objectives and the constraints to achieving them constitutes a key stage. Finally the role and evolutionary trend of each type of farming systems identified are evaluated in a synthesis at the regional agrarian system level. Taking into account the national agricultural policy and the international environment, priorities can be given to the various problems identified at the farm level to feed the second phase of on - farm design of appropriate technologies or socio - economic organizations for each type of agricultural produ