The death domain superfamily, composed of the death domain (DD), death effector domain (DED) and caspase recruitment domain (CARD) families of proteins, plays a pivotal role in signaling events that regulate apoptosis. This review compares and contrasts the ten superfamily members with known structures. In particular, the two heterodimerization modes described to date, the CARD-CARD interaction between human Apaf-1 and procaspase 9, and the DD-DD interaction between Drosophila Pelle and Tube, are examined. The dimerization modes are strikingly different and, importantly, are not mutually exclusive. In fact, a trimer can be formed using both interactions.