The present disclosure relates to a class of engineered polypeptides having a binding affinity for the neonatal Fc receptor (in the following referred to as FcRn), and provides an FcRn binding polypeptide comprising the sequence EX2X3X4AX6X7 EIRWLPNL X16X17X18QRX21 AFIX25X26LX28X29 (SEQ ID NO: 1075). The present disclosure also relates to the use of such an FcRn binding polypeptide as an agent for modifying pharmacokinetic and pharmacodynamic properties and as a therapeutic agent.