Tetrazine reagents is a class of click chemistry labeling reagents containing a reactive tetrazine group, Tetrazine reagent is highly reactive with TCO (trans-cycloctene) in an inverse electron demand Diels Alder reaction followed by a retro-Diels Alder reaction to eliminate nitrogen gas. This is a very quick reaction for bioconjugation at low concentrations in labeling live cells, molecular imaging and other bioconjugation applications.