The arbitrator’s responsibility with regard to illegality-based objections invites him or her to reconcile two imperatives, one, in the interest of investors, to preserve their access to arbitral tribunals as provided for in investment treaties, the other, in the interest of the host State, to ensure that such access is not to the detriment of investors’ compliance with local laws and, more generally, with international public policy. This article is divided into two parts. The first looks at the various ways in which the illegality of investment is sanctioned. This might be effected by depriving the investor of the procedural protection of the treaty — by declining jurisdiction or finding the claims inadmissible — or at the merits stage, when examining the substantial protections offered by the treaty. The second part then attempts to explain the influence that national criminal proceedings may have on arbitral proceedings. The latter, although not asked to determine the investor’s guilt, may indeed have to decide on requests to stay the proceedings, on objections based on the presumption of innocence and on questions pertaining to the admission of evidence of illegal acts.