When two human ova fail to fully separate during pregnancy, the result is conjoined twins. The twins may be connected by ligament, bone, or just flesh, and they often share organs, but what captures most people's interest is whether the twins share sensations, thoughts and even souls. This encyclopedia presents entries on conjoined twins throughout history, the biological causes and effects of twins being born conjoined, and ethical issues such as self-support and separation surgery. It also includes entries on the modern standardized terminology used when discussing conjoined twins, the categories into which conjoined twins have been sorted, doctors past and present who have performed separation surgeries, and hospitals that are known for the separation of conjoined twins. This also covers fraudulent conjoined twins and fictional ones in books written by such authors as Mark Twain and Vladimir Nabakov. Other entries cover relevant films, websites, and institutions.