  We consider the inverse problem of determining the isotropic inhomogeneous
electromagnetic coefficients of the non-stationary Maxwell equations in a
bounded domain of R^3, from a finite number of boundary measurements. Our main
result is a H\"older stability estimate for the inverse problem, where the
measurements are exerted only in some boundary components. For it, we prove a
global Carleman estimate for the heterogeneous Maxwell's system with boundary
conditions.
