If supersymmetry is realized non-linearly as is the case when auxiliary fie
lds are eliminated and/or one works in the Wess-Zumino gauge it is usually
incorporated in terms of BRS transformations and Slavnov-Taylor identities.
On the vertex functional susy transformations act even non-locally. Furthe
rmore, the gauge fixing term breaks supersymmetry. In the present paper we
clarify in which sense supersymmetry is still a symmetry of the system and
how it is realized on the level of quantum fields. We treat the Wess-Zumino
model as an example for chiral models, SQED and massless SYM as prototypes
of gauge theories. (C) 2001 Elsevier Science B.V. All rights reserved.