This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book provides a broad range of topics related to nitrogen (N) uptake in agricultural systems from laboratory, to field, to watershed scales, and they outline the current status of research and modeling in plant N uptake. It presents the state-of-the-science N-fixation submodel in the CROPGRO model, whereas most other agricultural system models assume the difference between legume N demand and N uptake as N fixation. The book identifies two different transporters in the winter oilseed rape in a N study, each with a constitutive and an inducible component. It provides a thoughtful review of the interactions among various factors contributing to N uptake in native ecosystems, such as the root/shoot ratio, root longevity, root architecture, and mycorrhizal colonization. The book addresses the mechanisms and extent of N uptake in the form of soil dissolved organic N.