Bio-orthogonal chemistry enables the reaction of two reagents to occur in the presence of a myriad of other reactive groups present in the cellular environment. We are developing new bio-orthogonal reactions that are fast, produce a single product (i.e., no regioisomers) and can be used in the presence of other bio-orthogonal reagents. These reactions are being used to prepare protein and nucleic acid-based bioconjugates and as chemical probes in cell biology.