The standard model of particle physics is invariant under a variety of continuous symmetry operations, including translations, Lorentz transformations, and gauge transformations. The model is also invariant under the action of the product CPT of charge conjugation C, parity reflection P, and time reversal T. Indeed, CPT symmetry is known to be a characteristic of all local relativistic field theories of point particles . It has been experimentally tested to high accuracy in a variety of situations . The general validity of CPT symmetry for particle theories and the existence of high-precision tests means CPT breaking is an interesting candidate experimental signal for new physics beyond the standard model, such as might emerge in the context of string theory [3, 4, 5]. In a talk  delivered at the previous meeting in this series (Orbis Scientiae 1997-I), I discussed the possibility that CPT and Lorentz symmetry might be broken in nature by effects emerging from a fundamental theory beyond the standard model. String theory, which currently represents the most promising framework for a consistent quantum theory of gravity incorporating the known particles and interactions, is a candidate theory in which effects of this type might occur. The point is that strings are extended objects, so the standard axioms underlying proofs of CPT invariance are inappropriate. In fact, it is known that spontaneous CPT and Lorentz violation can occur in the context of string theory [3, 7].