PEG Azide is a class of PEG attached click chemistry reagents that are widely used in bioconjugation, labeling and chemical modification due to its mild reaction conditions, fast speed and biocompatibility. Tha Azide group (N3) enables click chemistry with an alkyne to yield a stable triazole linkage. For the structure constrained alkynes such as BCN (bicyclo[6.1.0]nonyne) , DBCO (Dibenzocyclooctyne), no copper catalyst was needed for the reaction. The attraction of the azide functionality is in its high selectivity and stability under most conditions. These properties are especially useful where other conjugating functionalities have to be used due to their limited stability or when the careful control of variables, such as pH, are needed to ensure high yielding reactions. PEG is hydrophilic and improves the solubility of the reagent as well as the attached molecule. Choosing a different length of PEG can also help finetune the conjugated molecule s DMPK and other properties.