A method for modifying a polypeptide having at least one reactive thiol group, the method comprising contacting the polypeptide with a functionalising reagent which comprises a nitrogen containing heterocyclic aromatic ring having a vinyl substituent capable of reacting with the at least one thiol group of the polypeptide, wherein the functionalising reagent is covalently linked to a glycan group, so that the vinyl substituent of the functionalising reagent reacts with the thiol group of the polypeptide, thereby to covalently link the glycan group to the polypeptide, characterised in that the functionalising reagent is represented by one of the general formulae: or wherein: at least R 1, and optionally R 2, are covalently linked to a glycan group; and further wherein: R 1, and optionally R 2, are independently selected from: -(CH 2) n -Z-(CH 2) o -C -R, where Z is S or NH, n is 0 to 10, o is 0 to 10 and where R is a hydrogen, a poly(alkylene glycol) molecule or a glycan group; or -(CH 2) n -Z-(CH 2) o -R, where Z is S or NH, n is 0 to 10, o is 0 to 10 and where R is a hydrogen, a poly(alkylene glycol) molecule or a glycan group; or -C -Z-(CH 2) where Z is S or NH, o is 0 to 10 and where R is a hydrogen, a poly(alkylene glycol) molecule or a glycan group; or -(CH 2) n -Z-C -(CH 2) o -R, where Z is S or NH, n is 0 to 10, o is 0 to 10 and where R is a hydrogen, a poly(alkylene glycol) molecule or a glycan group; or -Y-(CH 2) n -Z-C -(CH 2) o -R, where Y is an aryl group, S or NH, Z is NH, n is 0 to 10, o is 0 to 10 and where R is a hydrogen, a poly(alkylene glycol) molecule or a glycan group; or -Y-R, where Y is S, R is a hydrogen, a glycan, a poly(alkylene glycol) molecule, aryl or C 1-10 alkyl; with the proviso that when R 2 is not linked to a poly(alkylene glycol) molecule or a glycan group, it may additionally be selected from hydrogen or an electron withdrawing group, such as halogen (F, Cl, or Br), -NO 2, -CO 2 H, -CO 2 R 4, COR 4, -CHO, -CN, -CF 3, -SO 2 NR 4 R 5, alkyl or phenyl, where R 4 and R 5 are independently selected from hydrogen or C 1-10 alkyl. at least R 1, and optionally R 2, are covalently linked to a glycan group; R 1, and optionally R 2, are independently selected from: -(CH 2) n -Z-(CH 2) o -C -R, where Z is S or NH, n is 0 to 10, o is 0 to 10 and where R is a hydrogen, a poly(alkylene glycol) molecule or a glycan group; or -(CH 2) n -Z-(CH 2) o -R, where Z is S or NH, n is 0 to 10, o is 0 to 10 and where R is a hydrogen, a poly(alkylene glycol) molecule or a glycan group; or -C -Z-(CH 2) where Z is S or NH, o is 0 to 10 and where R is a hydrogen, a poly(alkylene glycol) molecule or a glycan group; or -(CH 2) n -Z-C -(CH 2) o -R, where Z is S or NH, n is 0 to 10, o is 0 to 10 and where R is a hydrogen, a poly(alkylene glycol) molecule or a glycan group; or -Y-(CH 2) n -Z-C -(CH 2) o -R, where Y is an aryl group, S or NH, Z is NH, n is 0 to 10, o is 0 to 10 and where R is a hydrogen, a poly(alkylene glycol) molecule or a glycan group; or -Y-R, where Y is S, R is a hydrogen, a glycan, a poly(alkylene glycol) molecule, aryl or C 1-10 alkyl; with the proviso that when R 2 is not linked to a poly(alkylene glycol) molecule or a glycan group, it may additionally be selected from hydrogen or an electron withdrawing group, such as halogen (F, Cl, or Br), -NO 2, -CO 2 H, -CO 2 R 4, COR 4, -CHO, -CN, -CF 3, -SO 2 NR 4 R 5, alkyl or phenyl, where R 4 and R 5 are independently selected from hydrogen or C 1-10 alkyl. -(CH 2) n -Z-(CH 2) o -C -R, where Z is S or NH, n is 0 to 10, o is 0 to 10 and where R is a hydrogen, a poly(alkylene glycol) molecule or a glycan group; or -(CH 2) n -Z-(CH 2) o -R, where Z is S or NH, n is 0 to 10, o is 0 to 10 and where R is a hydrogen, a poly(alkylene glycol) molecule or a glycan group; or -C -Z-(CH 2) where Z is S or NH, o is 0 to 10 and where R is a hydrogen, a poly(alkylene glycol) molecule or a glycan group; or -(CH 2) n -Z-C -(CH 2) o -R, where Z is S or NH, n is 0 to 10, o is 0 to 10 and where R is a hydrogen, a poly(alkylene glycol) molecule or a glycan group; or -Y-(CH 2) n -Z-C -(CH 2) o -R, where Y is an aryl group, S or NH, Z is NH, n is 0 to 10, o is 0 to 10 and where R is a hydrogen, a poly(alkylene glycol) molecule or a glycan group; or -Y-R, where Y is S, R is a hydrogen, a glycan, a poly(alkylene glycol) molecule, aryl or C 1-10 alkyl;