There is no loss of generality in using light-front relativistic quantum theories. For systems of a finite number of degrees of freedom there are explicit S -matrix-preserving unitary transformations that transform theories with light-front kinematic subgroups to equivalent theories with instant-form or point-form kinematic subgroups. One expects that this is true in quantum field theory, although establishing the equivalence requires a nonperturbative definition of the theories in different forms of dynamics.