Nitrogen use efficiency is a complex trait, both physiologically and genetically and this thesis contributes to its detailed analysis. This research is highly relevant to ongoing efforts to increase productivity at low nitrogen input and to reduce negative environmental impact at high nitrogen input. The study consisted of an improved phenotyping strategy for above ground and below ground development of the potato crop using eco-physiological model approaches and QTL analysis on the basis of that detailed phenotyping work. This approach yielded many agronomically relevant variables that are affected by genotype and nitrogen management. These variables can be considered new traits, amenable to selection and many QTLs for these traits were identified using a diploid outcrossing population and a population of tetraploid cultivars. This approach integrated agronomy, crop physiology and genetic analysis towards more conscious breeding processes.