A method for glycosylating a therapeutic polypeptide, wherein the method comprises forming a covalent conjugate between a sugar acceptor and a 3-fluorosialic acid compound, the method comprising contacting the sugar acceptor, the 3-fluoro sialic acid compound and an enzyme capable of transferring the 3-fluoro sialic acid compound to the sugar acceptor, the contacting step taking place under conditions suitable for the transfer and covalent bonding of the 3-fluoro sialic acid compound to the sugar acceptor, wherein the 3-fluoro sialic acid compound does not comprise a cytidine monophosphate group, and wherein the sugar acceptor is present on, or for linkage to, the therapeutic polypeptide, and, wherein the 3-fluoro sialic acid compound is represented by general formula: wherein: Y 1 is selected from -0-, -S-, or -NR-, wherein R is independently selected from H, C 1-7 alkyl, C 3-10 heterocyclyl, or C 5-20 aryl; R 1 is a good leaving group capable of supporting and stabilising a negative charge, with the proviso that it is not a cytidine monophosphate group; X 1 is -CO 2 R, wherein R is as defined above; R 2 is selected from H, OH or halide; R 3 and R 4 are each independently selected from H, -OR, -NR 2 or -Z 1 (CH 2) m Z 2, where R is as defined above, Z 1 is selected from - 0-, -NR-, -CR 2 - and -S-, m is from 0 to 5 and Z 2 is selected from -OR, -NR 2 or -CN; with the proviso that R 3 and R 4 cannot both be H; R 5 is H; R 6 is selected from C 1-7 alkyl; C 1-7 hydroxyalkyl, C 1-7 amino alkyl or C 1-7 thioalkyl; R 7 is a group of formula: wherein Y 2 is selected from N, O, S, and CH; Z 3 is selected from H, hydroxyl, halide, C 1-7 alkyl, C 1-7 aminoalkyl, C 1-7 hydroxyalkyl, or C 1-7 thioalkyl; R 9 and R 10 are independently selected from H, hydroxyl, C 1-7 hydroxyalkyl, C 1-7 alkyl, C 5-20 aryl, C Z 4, wherein Z 4 is selected from C 1-7 alkyl or C 5-20 aryl, with the proviso that if Y 2 is 0 or S, R 10 is absent; or wherein R 4 is other than hydroxyl, R 7 may additionally be C 1-7 hydroxyalkyl; R 8 is hydrogen; or an oligomer of two or more molecules of formula; and stereoisomers, tautomeric forms, salts, solvates, or chemically protected forms thereof, wherein the term C 1-7 pertains to a monovalent moiety obtained by removing a hydrogen atom from a C 1-7 hydrocarbon, which may be aliphatic or alicyclic, or a combination thereof, and wherein the sugar acceptor is part of a glycosylation structure, the glycosylation structure comprises a naturally occurring or synthetic polysaccharide and which comprises the step of linking the conjugate to the therapeutic polypeptide when the glycosylation structure is not linked to the therapeutic polypeptide. Y 1 is selected from -0-, -S-, or -NR-, wherein R is independently selected from H, C 1-7 alkyl, C 3-10 heterocyclyl, or C 5-20 aryl; R 1 is a good leaving group capable of supporting and stabilising a negative charge, with the proviso that it is not a cytidine monophosphate group; X 1 is -CO 2 R, wherein R is as defined above; R 2 is selected from H, OH or halide; R 3 and R 4 are each independently selected from H, -OR, -NR 2 or -Z 1 (CH 2) m Z 2, where R is as defined above, Z 1 is selected from - 0-, -NR-, -CR 2 - and -S-, m is from 0 to 5 and Z 2 is selected from -OR, -NR 2 or -CN; with the proviso that R 3 and R 4 cannot both be H; R 5 is H; R 6 is selected from C 1-7 alkyl; C 1-7 hydroxyalkyl, C 1-7 amino alkyl or C 1-7 thioalkyl; R 7 is a group of formula: wherein Y 2 is selected from N, O, S, and CH; Z 3 is selected from H, hydroxyl, halide, C 1-7 alkyl, C 1-7 aminoalkyl, C 1-7 hydroxyalkyl, or C 1-7 thioalkyl; R 9 and R 10 are independently selected from H, hydroxyl, C 1-7 hydroxyalkyl, C 1-7 alkyl, C 5-20 aryl, C Z 4, wherein Z 4 is selected from C 1-7 alkyl or C 5-20 aryl, with the proviso that if Y 2 is 0 or S, R 10 is absent; or wherein R 4 is other than hydroxyl, R 7 may additionally be C 1-7 hydroxyalkyl; R 8 is hydrogen;