This paper uses Law & Economics as a tool of legal scholarship to analyse the concepts of good faith and contractual justice. Good faith is analysed as the exact opposite of opportunism. On opportunism there is a reasonably well-developed economic literature. Opportunism is present where, in a two or multi-party relationship, one of the participants schemes, by resorting to stealth or to force, to change in his or her favour the division of joint gains flowing from the relationship that each participant could normally look forward to when the relationship was set up. The win-win character that contracts and other relationships must have to be viable is thereby thrown out of kilter. The risk of becoming the victim of opportunism, of "being had"", by such schemes leads economic actors to take precautions that are costly and reduce the size of markets. Law may render useful services by combating opportunism in all its many forms, where this can be done cost-effectively and at lower cost than participants themselves could attain. In part, this defence is assured through a host of specific concepts found throughout the Civil Code. But cases may arise where none of these specific concepts can comfortably be applied. For such cases, an open-ended concept of last resort is needed. Good faith is such a concept. To ensure the certainty of the law, good faith, the anti-opportunism tool of last resort but with fuzzy boundaries, must be used only where no specific concept will do. Its use implies that over time new more specific concepts will crystallise as decisions accumulate. These new specific concepts should then go on lead an autonomous existence in the Code. Good faith, as the absence of opportunism, remains the principle underlying all of contract law and company law."