  Quantum Field Theory with fields as Operator Valued Distributions with
adequate test functions, -the basis of Epstein-Glaser approach known now as
Causal Perturbation Theory-, is recalled. Its recent revival is due to new
developments in understanding its renormalization structure, which was a major
and somehow fatal disease to its widespread use in the seventies. In keeping
with the usual way of definition of integrals of differential forms, fields are
defined through integrals over the whole manifold, which are given an
atlas-independent meaning with the help of the partition of unity. Using such
partition of unity test functions turns out to be the key to the fulfilment of
the Poincar\'e commutator algebra as well as to provide a direct Lorentz
invariant scheme to the Epstein-Glaser extension procedure of singular
distributions. These test functions also simplify the analysis of QFT behaviour
both in the UV and IR domains, leaving only a finite renormalization at a point
related to the arbitrary scale present in the test functions. Some well known
UV and IR cases are examplified. Finally the possible implementation of
Epstein-Glaser approach in light-front field theory is discussed, focussing on
the intrinsic non-pertubative character of the initial light-cone interaction
Hamiltonian and on the expected benefits of a divergence-free procedure with
only finite RG-analysis on physical observables in the end.
