The transnational investment reality is structured and functions like an ecosystem, and an ecosystem model is the best approach to map and understand it. Following this approach this work studies the oil and gas transnational investment reality, with the oil and gas transnational investment project as its focal element — thus, it studies the oil and gas transnational investment project ecosystem. The main objective is to propose measures that can help host States (HSs) conclude better deals in transnational investment project ecosystems. The quest for answers to the question of how HSs can conclude better deals in transnational investment project ecosystems involves an exploration of the sources of bad deals in order to understand why (causes) — but also how (mechanism and features) — HSs conclude bad deals. Such exercise is conducted in three interconnected fronts: negotiation of oil and gas transnational investment contracts; international investment law; and national investment law and policy. After that, proposals to remedy the identified problems are presented, answering the main question of this work. On the negotiation front, it was found that the complexity inherent to the structure and functions of HSs and of TNCs have the potential of negatively influencing the negotiation process and its outcomes for HSs. In the same vein, the bargaining power and interests of transnational financial institutions may weaken the HSs' position in the negotiation process. The general dynamics of the bargaining power, explained by theories such as the obsolescing bargain theory, seem to play a detrimental role in the achievement of better deals by HSs in entry negotiations. As to the different contractual models, they hold no inherent value, but all depend on the application of the right model with the appropriate clauses to the right circumstances. On the international investment law front, different versions, views and interpretations of international investment law are analyzed, exploring their interplay with transnational investment contracts, and the potential impact on the value of the HS's overall deal in the oil and gas transnational investment project ecosystem. The serious incompleteness of international investment law, the lack of consensus over crucial issues of this field of law, and the vague, standard-based nature of most investment treatment clauses are the main risks for the value of HSs' deals, as they often lead to unforeseen, adverse interpretations by investor-State dispute settlement bodies, raising the costs of concluding those instruments. On the national investment law and policy front, inadequate regulation of transfer pricing issues — which opens the door for "abusive" profit-shifting operations by TNCs — is one of the major contributors to bad deals in oil and gas transnational investment ecosystems. Another factor, or rather, feature of bad deals is inadequate protection of host populations' rights and interests. On this aspect, it is submitted that the conclusion of a deal with negligence or violation of host populations' rights and interests is, in extreme circumstances, tantamount to a bad deal, as it represents a negation of one of the basic functions of the State the pursuit of its people's well-being. The proposals to address the problems found mainly focus, first, on capacity building and transparency, improvement of negotiation strategy, and legal empowerment of host populations. Secondly, the work presents proposals on the reform of international investment law, submitting a model of reform via existing regional organizations. The proposed reform agenda encompasses, inter alia, reducing ambiguities in the design of international investment treaties and preserving the right of HSs to regulate on public interest issues. Finally, to deal with the challenges posed by complex transfer pricing operations, the adoption (with the necessary caution) of advanced pricing agreements to complement the prevalent tax administration audits and mutual agreement procedures is endorsed.