  In this work we define and study the relations between Lorentzian Manifolds
given by the diffeomorphisms which map causal future directed vectors onto
causal future directed vectors. This class of diffeomorphisms, called proper
causal relations, contains as a subset the well-known group of conformal
relations and are deeply linked to the so-called causal tensors. If two given
Lorentzian manifolds are in mutual proper causal relation then they are said to
be causally isomorphic: they are indistinguishable from the causal point of
view. Finally, the concept of causal transformations for Lorentzian manifolds
is introduced and its main mathematical properties briefly investigated.
