Despite the explosion of interest in heterocyclic azadienes, 1,2,3,5-tetrazines remain unexplored. Herein, the first general synthesis of this new class of heterocycles is disclosed. Its use in the preparation of a series of derivatives, and the first study of substituent effects on their cycloaddition reactivity, mode, and regioselectivity provide the foundation for future use. Their reactions with amidine, electron-rich, and strained dienophiles reveal unique fundamental reactivity patterns (4,6-dialkyl-1,2,3,5-tetrazines > 4,6-diaryl-1,2,3,5-tetrazines for amidines but slower with strained dienophiles), an exclusive C4/N1 mode of cycloaddition, and dominant alkyl versus aryl control on regioselectivity. An orthogonal reactivity of 1,2,3,5-tetrazines and the well-known isomeric 1,2,4,5-tetrazines is characterized, and detailed kinetic and mechanistic investigations of the remarkably fast reaction of 1,2,3,5-tetrazines with amidines, especially 4,6-dialkyl-1,2,3,5-tetrazines, established the mechanistic origins underlying the reactivity patterns and key features needed for future applications.