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 EX2 X3 X4 AX6 X7 EIRWLPNL X16X17 X18 QRX21 AFIX25 X26LX28 X29. 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.