If agricultural development is considered as the gradual changes occurring in the agricultural production processes that are assumed to be socially beneficial, the knowledge of these processes, their transformations in time as well as spatial adaptations is of primary importance. Especially because such a knowledge should be at the basis of any critical consideration or recommendation for action aiming at modifying the existing production processes and the on-going course of their changes in a given area. In FSRD, well-known concepts of the micro analysis are successfully used to understand, Evaluate and improve these production processes at the farm level. Meanwhile, at a larger scale and in order to identify the deep key transformations that affect in the long term the whole farms of a region, these useful concepts are not sufficient. Particularly, they do not permit the appraisal the conditions and consequences of the these determining evolutions and adaptations, especially their economic and social implications. To characterize such significant changes in the production processes at the regional or national levels, a more global concept is necessary that we call Agrarian System. At the crossroads between agro-ecology and agro-economy, the concept's main variables are the cultivated ecosystem and the instruments and means which are used to produce, maintain and exploit it, creating a specific mode of exploitation of the environment. Other essential interrelated variables that have also to be taken into account to explain the key transformations of agrarian systems are the division of labor between the various economic sectors, the relations of production, ownership and exchanges of the products, as well as the dominating political and cultural conditions and institutions. The paper presents how such a global concept can be used to: - propose a pertinent diagnosis on the present state and on-going evolution of a regional agriculture. - formulate adapted orientations for its future development, especially for the less-favoured groups of farmers. Recent studies on key changes in the rice-based agrarian systems of southern Thailand provide illustrations and concrete examples as well as demonstrate the relevance of this approach in the present state of Thai agriculture. It is also suggested that such study of the historical transformations and spatial adaptations of the agricultural production processes constitutes the subject to fan agricultural development science understood as a part of the global social development.