The adversarial legal procedure is a core characteristic of Anglo-American law. Adversarialism is widely heralded for serving the truth-seeking objective of the trial. The procedure’s core epistemic device is the dialectic exchange of vigorous partisan presentation. The truth is said to emerge through the sharp clash of proofs, that is, through zealous advocacy. Indeed, zealous representation is a cornerstone of American lawyering, and it amounts to a professional responsibility that lawyers owe their clients. This chapter examines zeal from a psychological perspective and characterizes it as the product of the adversarial bias. Acting in an adversarial role skews a wide array of factors in a self-serving manner, including judgments of the case, interpretations of the evidence, emotions toward the protagonists, judgments of the adversarial counterpart, and, crucially, the motivation to see the case come out in favor of one’s side. The research finds that the adversarial bias is undergirded by motivated coherence, a combination of the motivation to win and coherence-based reasoning. The chapter concludes that the adversarial bias propagates substantial harms. Inter alia, it impedes on lawyers’ ability to perform level-headed legal analysis, fosters a hyper-competitive and victory-focused legal process, and encourages biased testimony from both percipient and expert witnesses.