The present disclosure relates to a class of engineered polypeptides having a binding affinity for programmed death-ligand 1 (PD-L1), and provides a PD-L1 binding polypeptide comprising the sequence ERNX4AAX7EIL X11LPNLX16X17X18QX20 WAFIWX26LX28D. The present disclosure also relates to the use of such a PD-L1 binding polypeptide as a therapeutic, prognostic and/or diagnostic agent.