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 AdS5 × 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.Copyright line will be provided by the publisher The principle of causality stands at the foundation of all our understanding of physics. It cannot be relaxed without falling into paradoxes which, in the final analysis, consist either in the attempt to change the past conditions at the origin of the event or in the so-called bootstrap paradox, occurring when the effect is the cause of its own existence. The concept of causality is so important that it has been hardcoded into newtonian physics and special relativity. But as soon one takes into account the gravitational force, space and time become dynamical. Their evolution is driven by local equations and the equivalence principle ensures that the causality requirements of special relativity are locally respected. However, the gravitational field equations do not put any global constraint on the topology of time, leaving the possibility for the existence of closed timelike curves (CTCs), seeds of the violation of causality. The global nature of CTCs is what makes them so elusive and difficult to treat.Essentially, CTCs can arise by two mechanisms, rotation (or dragging of the inertial frames) and effectively superluminal propagation. The most well-known example of non topologically trivial CTCs induced by rotation is given by the Gödel universe [1], a homogeneous and simply connected manifold described by the metricHere, φ is a periodic angular coordinate, whose orbits become timelike for large r due to the framedragging term in the metric and form therefore CTCs. The fact that these CTCs exist at all times is a reason one can invoke to discard this solution. Another example is the Kerr black hole that has harmless CTCs induced by its rotation, but they are hidden by the Cauchy horizon, which is unstable. The situation is worse in higher dimensions, where naked CTCs are a generic feature of Kerr black holes [2]. It should be noted that supersymmetry does not help to rule out chronological pathologies [3], nor does the lifting to higher dimensional supergravities [4]. The other mechanism to create CTCs is to construct geometries allowing for effectively superluminal propagation of signals. For example, Gott's time machine [5] consists in two parallel cosmic strings in relative motion. The effect of the cosmic strings on the spacetime is to create conical defects, which can be cleverly exploited to build CTCs using the properties of Lorentz transformations (see figure 1). Again,