Summary: The dynamic nature of proteins in physiological environment makes it even difficult to determine the roles of individual amino acids in the chemical and biological properties of proteins. The ability to introduce modifications on protein structures is extremely useful as it allows for exploration of protein functions at sub-molecular level and the possibility of various applications such as producing protein-enzyme conjugates. In protein chemistry, crosslinking reagents are widely used to study intermolecular interactions and to construct new structures. Mechanistically, crosslinking strategies fall into three categories: chemical crosslinking, enzymatic crosslinking and photocrosslinking.