A single chain insulin analogue comprising: the A-chain of human or animal insulin, or an analogue thereof which is a variant wherein (a) one amino acid residue is replaced by a naturally or non-naturally occurring amino acid, (b) the order of two amino acid residues is reversed, or (c) both (a) and (b) are present together; the B-chain of human or animal insulin, or an analogue thereof which is a variant wherein (a) one amino acid residue is replaced by a naturally or non-naturally occurring amino acid, (b) the order of two amino acid residues is reversed, or (c) both (a) and (b) are present together; one or more of: (i) an intermolecular disulfide bond between the cysteine in position 7 of the insulin A-chain and the cysteine in position 7 of the insulin B-chain; (ii) an intermolecular disulfide bond between the cysteine in position 20 of the insulin A-chain and the cysteine in position 19 of the insulin B-chain; (iii) an intramolecular disulfide bond between the cysteine in position 6 and the cysteine in position 11 of the insulin A-chain; and a further covalent link, L, between the carboxyl terminal amino acid of the A-chain of human or animal insulin, or analogue thereof, and the side chain of the lysine residue in position 29 of the B-chain of human or animal insulin, or analogue thereof, wherein L is a group comprising: (i) at least one group X; and/or (ii) at least one group Y; and/or (iii) at least one group Z; wherein: X is a group of formula 1a or 1b wherein d and e are each independently an integer selected from 0 to 10, and * and ** denote the points of attachment to the respective adjacent groups; Y is a group selected from the following: where I is 1, 2, 3, 4 or 5, and * and ** denote the points of attachment to the respective adjacent groups; and Z is a natural or unnatural amino acid; wherein said single chain insulin analogue performs the same action as human insulin in terms of glycemic control. the A-chain of human or animal insulin, or an analogue thereof which is a variant wherein (a) one amino acid residue is replaced by a naturally or non-naturally occurring amino acid, (b) the order of two amino acid residues is reversed, or (c) both (a) and (b) are present together; the B-chain of human or animal insulin, or an analogue thereof which is a variant wherein (a) one amino acid residue is replaced by a naturally or non-naturally occurring amino acid, (b) the order of two amino acid residues is reversed, or (c) both (a) and (b) are present together; one or more of: (i) an intermolecular disulfide bond between the cysteine in position 7 of the insulin A-chain and the cysteine in position 7 of the insulin B-chain; (ii) an intermolecular disulfide bond between the cysteine in position 20 of the insulin A-chain and the cysteine in position 19 of the insulin B-chain; (iii) an intramolecular disulfide bond between the cysteine in position 6 and the cysteine in position 11 of the insulin A-chain; and (i) an intermolecular disulfide bond between the cysteine in position 7 of the insulin A-chain and the cysteine in position 7 of the insulin B-chain; (ii) an intermolecular disulfide bond between the cysteine in position 20 of the insulin A-chain and the cysteine in position 19 of the insulin B-chain; (iii) an intramolecular disulfide bond between the cysteine in position 6 and the cysteine in position 11 of the insulin A-chain; and a further covalent link, L, between the carboxyl terminal amino acid of the A-chain of human or animal insulin, or analogue thereof, and the side chain of the lysine residue in position 29 of the B-chain of human or animal insulin, or analogue thereof, wherein L is a group comprising: (i) at least one group X; and/or (ii) at least one group Y; and/or (iii) at least one group Z; (i) at least one group X; and/or (ii) at least one group Y; and/or (iii) at least one group Z; X is a group of formula 1a or 1b wherein d and e are each independently an integer selected from 0 to 10, and * and ** denote the points of attachment to the respective adjacent groups; Y is a group selected from the following: where I is 1, 2, 3, 4 or 5, and * and ** denote the points of attachment to the respective adjacent groups; and Z is a natural or unnatural amino acid; wherein said single chain insulin analogue performs the same action as human insulin in terms of glycemic control.