On the Algebra of Ghost Fields

We study in detail the algebra of free ghost fields which we realize in a Hilbert-Fock space with positive metric. The investigation of causality clarifies the exact reason for the failure of the spin-statistics theorem and leads to the introduction of the Krein Operator. We study the charge algebra of the ghost fields which gives a representation of ${\rm gl}(2,{\cal C})$. The symmetries of the $S$-matrix in ghost space are pointed out.