  We present a general method for constructing consistent quantum field
theories with global symmetries. We start from a free non-interacting quantum
field theory with given global symmetries and we determine all consistent
perturbative quantum deformations assuming the construction is not obstructed
by anomalies. The method is established within the causal
Bogoliubov-Shirkov-Epstein-Glaser approach to perturbative quantum field theory
(which leads directly to a finite perturbative series and does not rely on an
intermediary regularization). Our construction can be regarded as a direct
implementation of Noether's method at the quantum level. We illustrate the
method by constructing the pure Yang-Mills theory (where the relevant global
symmetry is BRST symmetry), and the N=1 supersymmetric model of Wess and
Zumino. The whole construction is done before the so-called adiabatic limit is
taken. Thus, all considerations regarding symmetry, unitarity and anomalies are
well-defined even for massless theories.
