Contracts are the bedrock of countless everyday interactions, from employment to consumer agreements. However, they can create a significant power imbalance when one party is granted sole discretion to make critical decisions that affect both sides. This research project examines how English law addresses this risk of abuse. Through an analysis of landmark court judgments and academic debates, this project investigates the two primary legal tools judges use to control this one-sided power: the duty of rationality and the duty of good faith. The findings reveal a courageous judicial trend towards actively implying these duties to ensure decisions are fair, rational, and aligned with the contract’s purpose, even when the written contract is silent. This represents a significant shift from a rigid adherence to absolute contractual freedom towards a greater concern for fairness and preventing exploitation. This research concludes that these parallel controls are essential for justice in modern contracting and argues for a more coherent legal standard to guide their application, ultimately helping to build a more equitable framework for commercial and personal relationships.