Despite Prolog's logic heritage and its use of theorem-proving unification and resolution operations, Prolog fails to qualify as a full general-purpose theorem-proving system. There are three main reasons: (1) many Prolog systems use an unsound unification algorithm, (2) Prolog's unbounded depth-first search strategy is incomplete, and (3) Prolog's inference system is not complete for non-Horn clauses. Nevertheless, Prolog is quite interesting from a theorem-proving standpoint because of its very high inference rate as compared to conventional theorem-proving programs. The objective of the Prolog Technology Theorem Prover (PTTP) is to overcome the deficiencies while retaining as fully as possible the high performance of well-engineered Prolog systems.