  I present the tensor computer algebra package FieldsX, which extends the xAct
suite of tensor algebra packages to perform computations in field theory with
fermions and gauge fields. This includes the standard tools of curved-space
$\gamma$ matrices, Fierz identities, invariant tensors on Lie algebras,
arbitrary gradings and left and right variational derivatives, as well as the
decomposition of spinor products into irreducible components following the
approach of d'Auria, Fr\'e, Maina and Regge [Annals Phys. 139 (1982) 93].
Lastly, it also includes functions to work with nilpotent differentials such as
the BV-BRST differential for (supersymmetric) gauge theories and to compute
their (relative) cohomologies, from which anomalies and gauge-invariant
operators can be determined. I illustrate the use of the package with the
example of $\mathcal{N}=1$ Super-Yang--Mills theory.
