  In this article we seek stability estimates in the inverse problem of
determining the potential or the velocity in a wave equation in an anisotropic
medium from measured Neumann boundary observations. This information is
enclosed in the dynamical Dirichlet-to-Neumann map associated to the wave
equation. We prove in dimension $n\geq 2$ that the knowledge of the
Dirichlet-to-Neumann map for the wave equation uniquely determines the electric
potential and we prove H\"older-type stability in determining the potential.
