This book deals with underlying basic concepts in relativity. The fundamental work of Stueckelberg, who formulated a consistent relativistic classical and quantum dynamics, generalized for application to many-body systems by Horwitz and Piron (SHP theory), is explained with emphasis on its conceptual content. The two-body bound state and scattering theory are also discussed. The ideas are involved in the Lindner experiment showing interference in time and the proposed experiment of Palacios et al. searching for the persistence of entanglement at unequal times is discussed. The meaning of the Newton-Wigner position operator and the Landau-Peierls construction in terms of relativistic dynamics is given. Finally, the embedding of the SHP theory into the framework of general relativity, providing a canonical structure with particle coordinates and momenta, is studied, carrying with it new concepts in relativistic dynamics.