S>The implications of group theory for the foundations of quaternionic quantum mechanics are discussed. It is proved that even in this case a relativistic description is possible. It is shown that the elementary systems for the Poincare group, as they appear in this realization, are in one-to-one correspondence with those of the usual complex quantum mechanics. The occurrence of charge conjugation related to space reflexion is discussed and some consequences on a TCP-relativistic theory are explored. The framework of a general relativistic quaternionic quantum mechanics is developed and the particular ties that exist between this theory and its complex analog are emphasized. The possible adequacy of a mixed complex-quaternionic description for isospin is discussed. (auth)