Entering into new and developing markets often involves partnership with economically and politically powerful persons with influence in that country.These joint ventures are not without peril, however.In international investment law, a co-venturer can expose partners to issues of potential illegality, such as whether influence peddling and bribery were among the animating purposes of that partnership.This chapter explores an emerging norm that molds over-arching principles of good faith and fair dealing into a very specific requirement that foreign investors engage in a high level of “due diligence” throughout their relationship with local partners or else be denied recourse to investor-state arbitration even if the host State injures the investment in violation of international law.While it was traditionally necessary to demonstrate that the foreign investor was complicit in any illegality the local partner may have committed, recent cases seem to be moving towards a stricter regime of “due diligence” that puts much of the onus on the foreign investor to ensure that national law is complied with, and that no fraud, corruption, or other illegality exists or persists. This chapter explores the implications of that expansion.