  The Lagrangian relativistic direct interaction theory in the various forms of
dynamics is formulated and its connections with the Fokker-type action theory
and with the constrained Hamiltonian mechanics are established. The motion of
classical two-particle system with relativistic direct interaction is analysed
within the framework of isotropic forms of dynamics in the two- and
four-dimensional space-time. Some relativistic exactly solvable
quantum-mechanical models are also discussed.
