Nitrogen synchrony refers to nitrogen (N) availability in the soil being closely aligned in space and time with the N requirements of crop plants. However, due to large temporal and spatial variability in the availability of soil N across regions, there is considerable potential for asynchrony between N supply and crop N demand. Synchronizing fertilizer N application to crop N requirements involves managing fertilizer N using strategies that lead to 'just-in-time N supply', i.e., the quantity is not excessive or deficient throughout the cropping season. Whether using split applications of fertilizer N, precision agriculture tools such as chlorophyll meters and canopy reflectance sensors, or fertilizer source modification, these strategies have been formulated over the last 20 to 30 years to concomitantly increase crop yield and the efficiency of fertilizer N use. This chapter explores the causes of N asynchrony and evaluates the strategies that have been developed to synchronize fertilizer N application to crop N requirements.