This Element provides a systematic analysis of Aristotle's theory of justice in exchange as developed in Nicomachean Ethics Book V. It examines Aristotle's distinction between voluntary exchanges (commutative justice) and involuntary transactions (corrective justice), explaining how proportional reciprocity in voluntary exchange secures an equitable balance between parties of different contributions, while corrective justice redresses unjust gains and losses in involuntary transgressions. Key concepts such as proportional reciprocity and the arithmetic mean are explored to show how Aristotle's vision of fair exchange underpins civic stability. The analysis also highlights Aristotle's notion of fairness (epieikeia) as a principled adjustment of rigid legal norms, illustrating how equitable judgement supplements strict justice to accommodate particular cases. Overall, the Element clarifies Aristotle's distinctions and principles, linking the ethics of equality and reciprocity to their practical application in law and commerce.