  The time slice axiom states that the observables which can be measured within
an arbitrarily small time interval suffice to predict all other observables.
While well known for free field theories where the validity of the time slice
axiom is an immediate consequence of the field equation it was not known
whether it also holds in generic interacting theories, the only exception being
certain superrenormalizable models in 2 dimensions. In this paper we prove that
the time slice axiom holds at least for scalar field theories within formal
renormalized perturbation theory.
