The chapter explains briefly about nitrogen cycle and the process involved in it. It tells about various processes such as Nitrification, Ammonification, Denitrification, Nitrate reductions, mineralization and immobilization. Nitrogen takes nine different forms in soil corresponding to different oxidative forms. Dinitrogen gas comprises 79%of our atmosphere and is by far the most abundant form of N in the atmosphere, but is unusable by most of the plants. Biological nitrogen fixation whereby nitrogen gas is transformed to organic nitrogen is the dominant natural process by which nitrogen enters into the soil. All subsequent soil N transformations are covered in this chapter.