Abstract: The study adopts a socio-environmental perspective to determine root causes of the widespread land degradation problem in many parts of the Southeast Asia. Based on a method combining visual assessment, focussed group discussion, information gathering from published literature and secondary data repositories and causal chain analysis for a showcase watershed in Thailand it is found that declining soil fertility and reduced water availability (restricting farmers` crop choice) are the immediate causes of land degradation. The causal factors responsible for soil fertility reduction are inappropriate agro-chemical use and soil erosion. The later is also responsible for silting up of the streams and reservoirs restricting the scarce source of irrigation. The identified root causes of these causal factors are many reflecting the complexity of issues affecting degradation. It is expected that understanding of the root causes of degradation will help develop priorities for action and set policy options to combat land degradation.