Nitrogen (N) is one of the most important nutrients in crop production; it is used worldwide to increase and maintain crop production and is considered a key element in maintaining the sustainability and economic viability of cropping systems across the world (Fixon and West 2002; Abbasi and Tahir 2012; Dillon et al. 2012). Before discussing the role of nitrogen in rice production, it is pertinent to define nutrition and discuss essential plant nutrients. Nutrition may be defined as the supply and absorption of chemical elements needed for growth and metabolism, and the chemical elements required by an organism are termed nutrients (Fageria and Baligar 2005a). Normal growth of higher plants requires 17 essential mineral nutrients (Figure 2.1). On the basis of quantity required by plants, nutrients are divided into macro and micro groups. The essential nutrients such as carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S) are known as macronutrients, whereas iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), molybdenum (Mo), chlorine (Cl), and nickel (Ni) are termed micronutrients. Further, the essentiality of silicon (Si), sodium (Na), vanadium (V), and cobalt (Co) has been considered but is not proven.