Among necessary plant nutrients, nitrogen is one of the most important yield-limiting nutrients, mainly responsible for determining yield and yield components in cereals and legumes. Sound nitrogen management for crops makes good economic sense. Optimal nitrogen fertilization is essential for achieving grain yield and quality targets and results in maximum economic return. Excess fertilization increases the risk of lodging and reduces net revenue. Good nitrogen management also makes good environmental sense. Excess fertilizer nitrogen application increases environmental losses of nitrogen, including nitrate leaching to groundwater and emissions of nitrous oxide, a greenhouse gas. The nitrogen supply for a cereal crop comes from fertilizer, but also from manure and mineralization. Mineralization is the release of plant available nitrogen from soil organic matter and crop residues as a result of soil microbial activity. The optimal fertilizer nitrogen rate varies from field-to-field and from year-to-year due to variation in both crop nitrogen demand and soil nitrogen supply. This book covers the critical aspects for the judicious use of nitrogen in cropping systems. This includes appropriate methods of nitrogen application, effective source and timing of application during crop growth cycles, use of an adequate application rate to avoid loss and reduce cost, use of nitrogen-efficient crop genotypes, and use of legumes that fix sufficient amounts of atmospheric nitrogen.