  Conditions which must be satisfied by the gauge-fixing fermion $\chi$ used in
the BRST quantisation of constrained systems are established. These ensure that
the extension of the Hamiltonian by the gauge-fixing term $[\Omega, \chi]$
(where $\Omega$ is the BRST charge) gives the correct path integral. (Lecture
given at the conference Constrained Dynamics and Quantum Gravity II, Santa
Margherita, Italy, September 1996)
