Manganese (Mn) is an essential micronutrient for all living organisms acting as a metal co-factor for approximately 6% of all known metalloenzymes. Photosynthesis is very sensitive to disturbances in plant availability of Mn, and both Mn deficiency and toxicity lead to distinct disturbances within the photosystems. However, plants are typically affected by Mn deficiency without any visual symptoms, and massive yield reductions are often observed if Mn fertilization is not applied. This chapter thoroughly presents and discusses the key functional roles of Mn in higher plants by introducing the classical studies and then focusing on the most recent literature. It covers all processes of Mn nutritional physiology from Mn mobilization in the rhizosphere, root uptake and translocation to the developing shoot, distribution and utilization within the cell, together with the recent advances in Mn sensing and regulation of Mn homeostasis. Finally, the authors describe the basic principles of diagnosing Mn deficiency and how efficient correction by soil and foliar fertilization can be performed in the field.