Synopsis Man made fertilizers have come to stay in the present century for increasing the output of agricultural production and to meet ever increasing demand of human population, which has been further accentuated by the limited availability of additional fertile farm land. Industrial nitrogen is heavily dependent on energy derived from fossil fuel which is getting depleted at a very fast rate. On the contrary, biological nitrogen fixation requiring nearly half the quantum of energy (needed for industrial fixation) and is dependent on energy from renewable resources such as products of photosynthesis and organic matter in the soil. The natural organic nitrogen provided by the biofertilizers to the agro-ecosystem do not pose any health risk to humans which is being seriously apprehended by the use of chemical and synthetic inorganic nitrogen. In view of escalating energy costs, it is essential for us to evolve and adopt a strategy of using biological fertilizers along with chemical fertilizers. This calls for adoption of integrated nutrient supply by using a judicious combination of chemical fertilizers and biofertilizers. The very objective of boosting crop productivity by chemical fertilizers is becoming self-defeating and unsustainable. An effort has been made in this book to provide an up-to date comprehensive account of fundamental as well as applied aspects related to biological N2-fixation through Azolla and Blue green algal bifertilizers.