Antibody polypeptides that specifically bind human CD40L are provided. The antibody polypeptides do not activate platelets. The antibody polypeptides are useful in the treatment of diseases involving CD40L activation, such as graft-related diseases and autoimmune diseases. The antibody polypeptides may be domain antibodies (dAbs) comprising a single VH or VK domain. The half-life of the antibody polypeptides may be increased by modifying the antibody polypeptides to be dual specific reagents that can also bind human serum albumin (HSA) or another antigen.