The invention relates to antibody polypeptides that monovalently bind CD40L. Antibody polypeptides that are monovalent for binding of CD40L can inhibit CD40L activity while avoiding potential undesirable effects that can occur with antibodies capable of divalent or multivalent binding of CD40L. In one aspect, a monovalent anti-CD40L antibody polypeptide consists of or comprises a single immunoglobulin variable domain that specifically binds and antagonizes the activity of CD40L, preferably without substantially agonizing CD40 activity. In another aspect, the monovalent anti-CD40L antibody polypeptide is a human antibody polypeptide. The invention further encompasses methods of antagonizing CD40/CD40L interactions in an individual and methods of treating diseases or disorders involving CD40/CD40L interactions, the methods involving administering a monovalent anti-CD40L antibody polypeptide to the individual.