A peptide ligand specific for CD137 comprising a polypeptide comprising at least three reactive groups, separated by at least two loop sequences, and a molecular scaffold which forms covalent bonds with the reactive groups of the polypeptide such that at least two polypeptide loops are formed on the molecular scaffold, characterised in that the peptide ligand comprises an amino acid sequence selected from: C i IEPGPFC ii YADPYMC iii (SEQ ID NO: 19); C i -I-E-E-G-Q-Y-C ii -X i -X 2 -D-X 3 -Y/Q/M-X 4 -C iii (SEQ ID NO: 20); C i -D-I-G-P-P-Y-C ii -Y-R/A-D-M/P-Y-M-C iii (SEQ ID NO: 21); C i -D-E-W-G-L-F/Y-C ii -I/F-P/A-H-S/P-D-C iii (SEQ ID NO: 22); C i IEEGQYC ii FADPY(Nle)C iii (SEQ ID NO: 23); C i IKEGQYC ii FADPY(Nle)C iii (SEQ ID NO: 24); C i IEKGQYC ii FADPY(Nle)C iii (SEQ ID NO: 25); C i IEE(D-K)QYC ii FADPY(Nle)C iii (SEQ ID NO: 26); C i IEEGKYC ii FADPY(Nle)C iii (SEQ ID NO: 27); C i IEEGQYC ii KADPY(Nle)C iii (SEQ ID NO: 28); C i IEEGQYC ii FADKY(Nle)C iii (SEQ ID NO: 29); C i IEEGQYC ii FADPYKC iii (SEQ ID NO: 30); C i -I/L/M/V-E/O/P/S-P/E/A-G-P/Q-Y/F-C ii -Y-A-O-P-Y/M-M/L/Y-C iii (SEQ ID NO: 41); and C i -X 5 -X 6 -X 7 -X 8 -X 9 -X 10 -C ii -X 11 -X 12 -D-X 13 -X 14 -X 15 -C iii (SEQ ID NO: 266); wherein C i, C ii and C iii represent first, second and third cysteine residues, respectively; X 1 -X 4 represent any amino acid residue; X 5 represents Ile, tBuAla or Chg; X 6 represents Glu, Pro, Asp, Lys, Aad, HyP or Oxa; X 7 represents Glu, Lys or Aad; X 8 represents Gly, D-Lys, D-Ala, L-Ala, D-Phe, D-Glu, D-Gln, D-Leu, D-Ser or D-Trp; X 9 represents Gln, Lys, Ala, Pro, 5,5-dmP, Oic, Oxa, HyP, Aib or Ac5c; X 10 represents Tyr, Phe, 3MePhe, 4MePhe, 4FPhe, 2Nal, 4MeOPhe or 4,4-BPA; X 11 represents Phe, Lys, 4MePhe, 2FPhe, 4FPhe, 4Pal, 4,4-BPA, 4tBuPhe, NO2Phe or 4BrPhe; X 12 represents Ala or Lys; X 13 represents Pro or Lys; X 14 represents Tyr or Lys; and X 15 represents Met, Lys, Nle, HLeu or Ahp, or a pharmaceutically acceptable salt thereof. C i IEPGPFC ii YADPYMC iii (SEQ ID NO: 19); C i -I-E-E-G-Q-Y-C ii -X i -X 2 -D-X 3 -Y/Q/M-X 4 -C iii (SEQ ID NO: 20); C i -D-I-G-P-P-Y-C ii -Y-R/A-D-M/P-Y-M-C iii (SEQ ID NO: 21); C i -D-E-W-G-L-F/Y-C ii -I/F-P/A-H-S/P-D-C iii (SEQ ID NO: 22); C i IEEGQYC ii FADPY(Nle)C iii (SEQ ID NO: 23); C i IKEGQYC ii FADPY(Nle)C iii (SEQ ID NO: 24); C i IEKGQYC ii FADPY(Nle)C iii (SEQ ID NO: 25); C i IEE(D-K)QYC ii FADPY(Nle)C iii (SEQ ID NO: 26); C i IEEGKYC ii FADPY(Nle)C iii (SEQ ID NO: 27); C i IEEGQYC ii KADPY(Nle)C iii (SEQ ID NO: 28); C i IEEGQYC ii FADKY(Nle)C iii (SEQ ID NO: 29); C i IEEGQYC ii FADPYKC iii (SEQ ID NO: 30); C i -I/L/M/V-E/O/P/S-P/E/A-G-P/Q-Y/F-C ii -Y-A-O-P-Y/M-M/L/Y-C iii (SEQ ID NO: 41); and C i -X 5 -X 6 -X 7 -X 8 -X 9 -X 10 -C ii -X 11 -X 12 -D-X 13 -X 14 -X 15 -C iii (SEQ ID NO: 266); wherein C i, C ii and C iii represent first, second and third cysteine residues, respectively; X 1 -X 4 represent any amino acid residue; X 5 represents Ile, tBuAla or Chg; X 6 represents Glu, Pro, Asp, Lys, Aad, HyP or Oxa; X 7 represents Glu, Lys or Aad; X 8 represents Gly, D-Lys, D-Ala, L-Ala, D-Phe, D-Glu, D-Gln, D-Leu, D-Ser or D-Trp; X 9 represents Gln, Lys, Ala, Pro, 5,5-dmP, Oic, Oxa, HyP, Aib or Ac5c; X 10 represents Tyr, Phe, 3MePhe, 4MePhe, 4FPhe, 2Nal, 4MeOPhe or 4,4-BPA; X 11 represents Phe, Lys, 4MePhe, 2FPhe, 4FPhe, 4Pal, 4,4-BPA, 4tBuPhe, NO2Phe or 4BrPhe; X 12 represents Ala or Lys; X 13 represents Pro or Lys; X 14 represents Tyr or Lys; and X 15 represents Met, Lys, Nle, HLeu or Ahp, or a pharmaceutically acceptable salt thereof. X 1 -X 4 represent any amino acid residue; X 5 represents Ile, tBuAla or Chg; X 6 represents Glu, Pro, Asp, Lys, Aad, HyP or Oxa; X 7 represents Glu, Lys or Aad; X 8 represents Gly, D-Lys, D-Ala, L-Ala, D-Phe, D-Glu, D-Gln, D-Leu, D-Ser or D-Trp; X 9 represents Gln, Lys, Ala, Pro, 5,5-dmP, Oic, Oxa, HyP, Aib or Ac5c; X 10 represents Tyr, Phe, 3MePhe, 4MePhe, 4FPhe, 2Nal, 4MeOPhe or 4,4-BPA; X 11 represents Phe, Lys, 4MePhe, 2FPhe, 4FPhe, 4Pal, 4,4-BPA, 4tBuPhe, NO2Phe or 4BrPhe; X 12 represents Ala or Lys; X 13 represents Pro or Lys; X 14 represents Tyr or Lys; and X 15 represents Met, Lys, Nle, HLeu or Ahp, or a pharmaceutically acceptable salt thereof.