  We review the issue of chronology protection and show how string theory can
solve it in the half BPS sector of AdS/CFT. According to the LLM prescription,
half BPS excitations of AdS_5 x S^5 geometries in type IIB string theory can be
mapped into free fermion configurations. We show that unitarity of the theory
describing these fermions is intimately related to the protection of the
chronology in the dual geometries.
