Sajjan Singh v. State of Rajasthan is a landmark judgment of the Supreme Court of India that introduced the concept of basic structure in the Indian constitutional law. The case was decided in 1965, and it has played a crucial role in shaping the Doctrine of Basic Structure, which is a judicial doctrine that limits the power of the Parliament to amend the Constitution of India.

The case arose out of a challenge to the Constitution (Seventh Amendment) Act, 1956, which amended the Constitution to enable the acquisition of the estates of the princely states in Rajasthan. The appellant, Sajjan Singh, was the Raja of Ratlam and he challenged the validity of the Act on the grounds that it violated his fundamental rights under Articles 14, 19, and 31 of the Constitution.

The Supreme Court, in its judgment, held that the Parliament has the power to amend the Constitution under Article 368, but this power is subject to certain limitations. The Court observed that the Constitution is not a mere legal document, but a social contract between the people of India and the government. Therefore, it cannot be amended in a way that destroys its essential features and fundamental principles.

The Court further held that the Constitution has certain basic features that cannot be altered by the Parliament, even through a constitutional amendment. These basic features include the sovereignty of the nation, the democratic form of government, and the rule of law.

The Court also held that the Parliament cannot amend any fundamental right, including the right to property, in a way that takes away the essence of that right. The Court observed that the fundamental rights are the cornerstone of the Constitution and are essential for the protection of the individual liberty and dignity of the citizens.

The Doctrine of Basic Structure, as formulated by the Supreme Court in the Sajjan Singh case, has played a crucial role in protecting the core principles of the Constitution of India from any amendment by the Parliament. It has helped to preserve the sovereignty, democracy, and rule of law of the Constitution, and has ensured that the fundamental rights of the citizens are protected.