  We study the Lagrangian structure of relativistic Vlasov systems, such as the
relativistic Vlasov-Poisson and the relativistic quasi-eletrostatic limit of
Vlasov-Maxwell equations. We show that renormalized solutions of these systems
are Lagrangian and that these notions of solution, in fact, coincide. As a
consequence, finite-energy solutions are shown to be transported by a global
flow. Moreover, we extend the notion of generalized solution for "effective"
densities and we prove its existence. Finally, under a higher integrability
assumption of the initial condition, we show that solutions have every energy
bounded, even in the gravitational case. These results extend to our setting
those obtained by Ambrosio, Colombo, and Figalli \cite{vlasovpoisson} for the
Vlasov-Poisson system; here, we analyse relativistic systems and we consider
the contribution of the magnetic force into the evolution equation.
