Gauge fixing and ghost variables

An approach to second-class constrained systems is presented which directly extends the Batalin-Fradkin-Vilkovisky formulation of first-class systems. This is achieved without any modification to the constraints. Within this approach the standard argument against the use of second-class constraints in the quantum theory no longer holds. These methods are applied to the bosonic string and a new approach to the sub-critical string is advocated. (orig.)