Many Nitrogen (N) recommendation systems are mainly based on yield expectation with a given N uptake. However, yield levels are highly variable between years and from field to field (and even within a field). Also the processes that determine the N supply from other sources than fertilizer are not sufficient predictable at the start of the growing season. Thus, the optimum N rate is not constant for a specific crop or field, but can vary substantially from cultivar to cultivar, site to site and year to year. The main reason for the variability between years are changing and unpredictable weather conditions, while differences within a field can mainly be correlated to soil conditions. Both factors influence in particular water and nutrient availability for the crops. The use of plant analysis as a method for N fertilizer recommendation is based on the idea that the plant itself is the best indicator for the N supply from any source within the growth period. The nitrogen status of plants reflects and integrates all effects in relation to varying weather conditions, which in turn determine nitrogen nutrition and crop growth. Prerequisites are a base dressing of fertilizer N and top dressings later in the growth period as well as easy to use plant analytical devices, which are delivering prompt results. In-field methods like the nitrate plant sap/petiole test, chlorophyllmeter measurements and optical measurements of crop canopy via remote sensing are regarded as most promising, because with these methods an adequate adjustment of the N fertilizer application strategy within the season is feasible. Field trial results show the advantage of N fertilization strategies based on in-season and real-time plant analysis compared to traditional and mainly static N recommendation schemes.