Historically, philosophers, naturalists and biologists alike have referred to the restoration of missing body parts after traumatic injury as regeneration. While still valid today, the concept of regeneration has expanded through the years to include a diverse set of phenomena. For instance, August Weisman considered physiological cell renewal to be regeneration and wrote so in a chapter dedicated to regeneration in his seminal 1893 book The Germ Plasm: 'the functions of certain organs depend on the fact that their parts continually undergo destruction, and are then correspondingly renewed. In this case it is the process of life itself, and not an external enemy, that destroys the life of a cell' . Soon after, TH Morgan would also attempt to refine the precision of the concept of regeneration by coining terms that distinguish between regeneration requiring cell proliferation (epimorphosis) and regeneration effected by tissue remodeling (morphallaxis) . Presently, regeneration is used to include multiple restorative processes manifested either as a result of physiological turnover (for example, the renewal of blood, skin and gut epithelial cells) or injury, and more recently has been used to define a branch of medical practice referred to as 'regenerative medicine'. Thus, rather than becoming more specific, the concept of regeneration has become much more general. This peculiarity can be attributed in great part to the fact that presently, and not unlike previous centuries, little unambiguous molecular, cellular, and evolutionary evidence exists to support a common or divergent mechanism controlling physiological and traumatic regeneration within and between species. That such diverse biological phenomena as adult neurogenesis and limb regeneration can be catalogued under the same umbrella is indicative of our limited mechanistic understanding of regenerative processes, and thus underscores how much more discovery research remains to be done.