The present disclosure relates to polypeptides which bind to human epidermal growth factor receptor 3 (HER3) and to use of such polypeptides in imaging and therapy. The disclosure provides an HER3 binding polypeptide comprising a HER3 binding motif, which motif consists of the amino acid sequence EKYX4AYX7EIW X11LPNLTX17X18QX20 AAFIGX26 LX28D (SEQ ID NO:110).