Abstract: Reagents and methods for functionalising polypeptides with moieties poly (alkylene glycol) molecules and glycan groups are disclosed that are based on a functionalising reagent which comprises a nitrogen containing heterocyclic aromatic ring having a vinyl substituent that is capable of reacting with one or more thiol groups that are naturally present, or have been introduced into, the polypeptide, for example by employing a thiol group of one or more cysteine residues. The functionalising reagent is covalently linked to a poly (alkylene glycol) molecule, such as a polyethylene glycol (PEG) molecule, or a glycan group so that the reaction between the vinyl group and the thiol group in the polypeptide covalently links the polypeptide to the poly (alkylene glycol) molecule and/or the glycan group.