Abstract: This invention provides binding proteins, including antibodies, antibody derivatives and antibody fragments, that specifically bind a CD154 (CD40L) protein. This invention also provides a chimeric, humanized or fully human antibody, antibody derivative or antibody fragment that specifically binds to an epitope to which a humanized Fab fragment comprising a variable heavy chain sequence according to SEQ ID NO: 1 and comprising a variable light chain sequence according to SEQ ID NO: 2 specifically binds. CD154 binding proteins of this invention may elicit reduced effector function relative to a second anti-CD154 antibody. CD154 binding proteins of this invention are useful in diagnostic and therapeutic methods, such as in the treatment and prevention of diseases including those that involve undesirable immune responses that are mediated by CD154-CD40 interactions.