The Gauge-Fixing Fermion in BRST Quantisation

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)