  The metric field of general relativity is almost fully determined by its
causal structure. Yet, in spin-foam models for quantum gravity, the role played
by the causal structure is still largely unexplored. The goal of this paper is
to clarify how causality is encoded in such models. The quest unveils the
physical meaning of the orientation of the two-complex and its role as a
dynamical variable. We propose a causal version of the EPRL spin-foam model and
discuss the role of the causal structure in the reconstruction of a
semiclassical spacetime geometry.
