Chalcogen-nitrogen chemistry deals with systems in which sulfur, selenium or tellurium is linked to a nitrogen nucleus. This chemical motif is a key component of different functional systems, ranging from inorganic materials and polymers to rationally designed catalysts, to bioinspired molecules and natural enzymes. The formation of a selenium-nitrogen bond, typically occurring upon condensation of an amine and the unstable selenenic acid and often leading to intramolecular cyclizations, and its disruption, mainly promoted by thiols, are rather common events in organic Se-catalyzed processes. In nature, along the mechanistic path of glutathione peroxidase, evidence of the formation of a cyclic intermediate characterized by the presence of a Se-N bond in highly oxidizing conditions has been reported and interpreted as a strategy to protect the selenoenzyme by overoxidation. Selenium is bonded to nitrogen also in the well-known ebselen, a selenenylamide with antioxidant, antimicrobic and cytoprotective activity and its formation/disruption has a crucial role for its drug action. Focusing on examples taken from selenium organic chemistry and biochemistry, the selenium-nitrogen bond is described and its strength and reactivity are quantified using accurate computational methods applied to model molecular systems. Significant trends show up when comparing to sulfur/tellurium-nitrogen bonds, reaffirming also in this context the peculiar and valuable role of selenium in chemistry and life.