Though there are numerous results for the existence and uniqueness of
Euler-Poisson equations in various situations, surprisingly, the continues
depending on the initial data has not been established so far. In a recent joint
work with U. Brauer we proved the continuity of the flow map for symmetric
hyperbolic systems and we applied it to Euler-Poisson equation. Previously,
the continuous depending on the initial data for symmetric hyperbolic was proved
by Kato, our method is based upon a new type of energy estimates. This method
will enable us to apply it for relativistic flows, including non--isentropic flows.