The question of why people should obey the law remains one of the central and continuing problems in jurisprudence and political philosophy. Obedience raises not only the question whether a rule is legally valid, but also whether citizens owe a moral duty to follow it. This paper examines why people obey law by separating legal validity from moral obligation. The paper discusses the major theories that have attempted to explain why people obey the law, including natural law, social contract theory, utilitarianism and legal positivism. It gives particular attention to H.L.A. Hart’s theory of legal obligation, especially his distinction between being “obliged” and being “under an obligation”, the internal point of view, and the practice theory of rules. Hart connects legal obligation with the acceptance of rules as standards of conduct within a legal system, yet he does not argue that a valid law automatically creates a moral duty to obey it. The paper then considers Hart’s idea of the minimum content of natural law and its practical importance for maintaining social order, security and cooperation. It examines the views of Joseph Raz and Lon Fuller to show different approaches to legal authority, legality and fidelity to law. The paper finally addresses the difficult issue of civil disobedience, particularly where valid laws are regarded as unjust. The examples of Gandhi’s Salt Satyagraha and Martin Luther King Jr.’s civil rights movement demonstrate that obedience to law may come into conflict with moral conscience. The paper concludes that no single reason applies in every situation, and that the duty to obey law depends upon the relationship between legal validity, social order, moral responsibility and the principles of justice.