Nitrogen deficiency is one of the major factors involved in declining soil fertility and land productivity in the Hindu Kush- Himalayan (HKH) region. However, use of chemical nitrogen fertilizer to replenish soil nitrogen is often not feasible for both practical and economic reasons, and in many cases is actually the cause of other environmental problems. One easy alternative, freely available to mountain farmers, is to use nitrogen fixing plants as an integral part of the agricultural system to increase soil nitrogen. However, to be able to exploit the potential of biological nitrogen fixation to the full, it is important first to understand the principles of application, the different types of plants involved, and the different possible uses of these different species in agriculture and forestry. This book provides a brief description of the principles of biological nitrogen fixation, the different types of nitrogen fixing plants, leguminous and non-leguminous, and the major potential usesof these plants to support agricultural development in the HKH region. Separate sections are devoted to various means of improving soil fertility, soil conservation, rehabilitation of degraded land, development of woody fodder, development of fuelwood crops, the use of terrace risers, and various other uses of nitrogen fixing plants. The unique characteristics of nitrogen fixing plants, including their ability to fix nitrogen from the air, to colonize very poor soils, and to act as pioneering plants, their fast growth, and their high content of protein, nitrogen and other nutrients, can be used to help achieve sustainable mountain development by meeting people's basic needs for fertilizers, fodder, and fuelwood, as well as providing an important source of improved nutrition.