  We discuss the notion of causality in Quantum Gravity in the context of
sum-over-histories approaches, in the absence therefore of any background time
parameter. In the spin foam formulation of Quantum Gravity, we identify the
appropriate causal structure in the orientation of the spin foam 2-complex and
the data that characterize it; we construct a generalised version of spin foam
models introducing an extra variable with the interpretation of proper time and
show that different ranges of integration for this proper time give two
separate classes of spin foam models: one corresponds to the spin foam models
currently studied, that are independent of the underlying orientation/causal
structure and are therefore interpreted as a-causal transition amplitudes; the
second corresponds to a general definition of causal or orientation dependent
spin foam models, interpreted as causal transition amplitudes or as the Quantum
Gravity analogue of the Feynman propagator of field theory, implying a notion
of ''timeless ordering''.
