The Constitution of India is the supreme law of India. It lays down the framework defining fundamental political principles, establishing the structure, procedures, powers and duties, of the government and spells out the fundamental rights, directive principles and duties of citizens. Passed by the Constituent Assembly on November 26, 1949, it came into effect on January 26, 1950. It declares the Union of India to be a sovereign, democratic republic, assuring its citizens of justice, equality, and liberty; the words "socialist", "secular" and "integrity" were added to the definition in 1976 by constitutional amendment. India celebrates the adoption of the constitution on January 26 each year as Republic Day. It is the longest written constitution of any sovereign nation in the world, containing 395 articles, 12 schedules and 94 amendments, for a total of 117,369 words in the English language version. Besides the English version, there is an official Hindi translation. Being the supreme law of the country, every law enacted by the government must conform to the constitution.

The Constitution was drafted by the Constituent Assembly, which was elected by the elected members of the provincial assemblies. Jawaharlal Nehru, C. Rajagopalachari, Rajendra Prasad, Sardar Vallabhbhai Patel, Maulana Abul Kalam Azad, Shyama Prasad Mukherjee and N R Ghosh [Nalini Ranjan Ghosh] were some important figures in the Assembly.
There were more than 30 members of the scheduled classes. Frank Anthony represented the Anglo-Indian community, and the Parsis were represented by H. P. Modi and R. K. Sidhwa. The Chairman of the Minorities Committee was Harendra Coomar Mookerjee, a distinguished Christian who represented all Christians other than Anglo-Indians. Ari Bahadur Gururng represented the Gorkha Community. Prominent jurists like Alladi Krishnaswamy Iyer, B. R. Ambedkar, Benegal Narsing Rau and K. M. Munshi Ganesh Mavlankar were also members of the Assembly. Sarojini Naidu, Hansa Mehta, Durgabai Deshmukh and Rajkumari Amrit Kaur were important women members. The first president of the Constituent Assembly was Sachidanand Sinha later, Rajendra Prasad was elected president of the Constituent Assembly. The members of the Constituent Assembly met for the first time in the year 1946 on December 9.

In the August 14, 1947 meeting of the Assembly, a proposal for forming various committees was presented. Such committees include Committee on Fundamental Rights, the Union Powers Committee and Union Constitution Committee. On August 29, 1947, the Drafting Committee was appointed, with Dr. Ambedkar as the Chairman along with six other members. A Draft Constitution was prepared by the committee and submitted to the Assembly on November 4, 1947.
The Assembly met, in sessions open to public, for 166 days, spread over a period of 2 years, 11 months and 18 days before adopting the Constitution.[3] After many deliberations and some modifications, the 308 members of the Assembly signed two hand-written copies of the document (one each in Hindi and English) on the January 24, 1950. Two days later, the Constitution of India became the law of all the Indian lands.
The Indian Constitution has undergone 108 amendments in its less than 60 years of enactment. The intellectual Dr Ambedkar,hailed as the Architect of the Indian Constitution, with hundreds of statues erected in his honor, studied at Columbia University and had the opportunity of studying the time tested American Constitution.

The basic form of the Union Government envisaged in the Constitution was introduced by Dr. Ambedkar as follows,

The Preamble states:

The word sovereign means supreme or independent. India is internally and externally sovereign - externally free from the control of any foreign power and internally, it has a free government which is directly elected by the people and makes laws that govern the people...

The word socialist was added to the Preamble by the 42nd amendment act of 1976, during the emergency. It implies social equality, and does not connote any economic or political ideology. Social equality in this context means the absence of discrimination on the grounds only of caste, colour, creed, sex, religion, or language. Under social equality, everyone has equal status and opportunities. Economic equality in this context means that the government will endeavor to make the opportunities available to its citizens equitable, and each citizen is to have every right to improve his or her condition, on his or her own efforts and merits. This is not to emphasise a commitment towards the formation of a welfare state, as evidenced by the Indian government's decision to open public business schools, known formally as the Indian Institutes of Management, around the same time as the enactment of this amendment.

The word secular was inserted into the Preamble by the 42nd amendment act of 1976, during emergency. It implies equality of all religions and religious tolerance. India, therefore does not have an official state religion. Every person has the right to preach, practice and propagate any religion they choose. The government must not favor or discriminate against any religion. It must treat all religions with equal respect. All citizens, irrespective of their religious beliefs are equal in the eyes of law. No religious instruction is imparted in government or government-aided schools. Nevertheless, general information about all established world religions is imparted as part of the course in Sociology, without giving any importance to any one religion or the others. The content presents the basic/fundamental information with regards to the fundamental beliefs, social values and main practices and festivals of each established world religions. The Supreme Court in S.R Bommai v. Union of India held that secularism was an integral part of the basic structure of the constitution.

India is a democracy. The people of India elect their governments at all levels (Union, State and local) by a system of universal adult franchise; popularly known as 'One man one vote'. Every citizen of India, who is 18 years of age and above and not otherwise debarred by law, is entitled to vote. Every citizen enjoys this right without any discrimination on the basis of caste, creed, colour, sex, religion or education.

As opposed to a monarchy, in which the head of state is appointed on hereditary basis for a lifetime or until he/she abdicates from the throne, a democratic republic is an entity in which the head of state is elected, directly or indirectly, for a fixed tenure. The President of India is elected by an electoral college for a term of five years. The Post of the President Of India is not hereditary. Every citizen of India is eligible to become the President of the country.
Preamble plays pivotal role when there is ambiguity in provisions of any Article or interpretation becomes confusing, spirit of preamble becomes guiding factor. Preamble is stem, root and source of constitution.