Good faith is a doctrine that is readily accepted in legal systems. Yet, its distinct meaning has always been elusive. Ideas such as justice and equity are omnipresent in the law. Good faith is counted among such ideas. Their function has been to provide a corrective approach in situations where the strict application of the law has unacceptable results. They are also used to support a decision-maker's conclusions on difficult issues where other solutions are equally possible. In these situations, good faith and similar ideas become rationalizations for the results arrived at. Defined or explained in this way, their existence in the law may be desirable. But put differently, these nebulous doctrines exist in the law to aid in finding subjective solutions to difficult issues, which may come to be justified through the use of such lofty notions. The latter conclusion immediately invites the criticism that the doctrine of good faith is capable of manipulation in order to justify a variety of inconsistent results. Critics argue that the subjectivity inherent in these concepts makes their use of doubtful significance. They have such a variable meaning that they could be used to support a variety of conclusions and befuddle the law in its search for certainty. Having in itself no power to lead to conclusions, the purpose of a concept such as good faith may be misguided: that of justifying conflicting solutions to difficult problems. This book demonstrates the range of scholarly views applicable to good faith in international investment law and the questions that remain to be answered.