  We explain how one-loop amplitudes with massive fermions can be computed
using only on-shell information. We first use the spinor-helicity formalism in
six dimensions to perform generalised unitarity cuts in $d$ dimensions. We then
show that divergent wavefunction cuts can be avoided, and the remaining
ambiguities in the renormalised amplitudes can be fixed, by matching to
universal infrared poles in $4-2\epsilon$ dimensions and ultraviolet poles in
$6-2\epsilon$ dimensions. In the latter case we construct an effective
Lagrangian in six dimensions and reduce the additional constraint to an
on-shell tree-level computation.
