  We present the fermionic universal one--loop effective action obtained by
integrating out heavy vector--like fermions at one loop using functional
techniques. Even though previous approaches are able to handle integrating out
heavy fermions with non--chiral interactions, i.e. vanishing $\gamma^5$
interaction terms, the computations proceed in a tedious manner that obscures a
physical interpretation. We show how directly tackling the fermionic functional
determinant not only allows for a much simpler and transparent computation, but
is also able to account for chiral interaction terms in a simple, algorithmic
way. Finally, we apply the obtained results to integrate out at one loop the
vector--like fermions appearing in a toy model and in a fermionic model that
exhibits strong cosmological phase transitions.
