The various modes of acquisition of citizenship are prescribed by the Citizenship Act, 1955.
(1) By birth: Citizenship is conferred on those born in India on or after January 26, 1950 but before June 30, 1987; and those born on or after July 1, 1987 if either of his parents was a citizen of India at the time of his birth. (the latter provision was added by an amendment to the Citizenship Act in 1986 in the wake of refugee influx from Bangladesh, Sri Lanka etc.)
(2) By Descent: A person born outside India or after January 16, 1950 shall be a citizen of India if his father is a citizen of India at the time of the person’s birth.
(the above two categories are natural citizens)
(3) By registration: a person can acquire citizenship by registering himself before the prescribed authority, provided he is of Indian origin and has been resident in India for five years (earlier it was six months) immediately before making an application.
(4) By naturalisation: a foreigner can acquire citizenship on application for naturalisation to the government of India. A person acquiring citizenship by naturalisation has to take an oath of allegiance to the Constitution of India.
(5) By incorporation of territory: if any new territory becomes a part of India, the Government of India specifies the persons of that territory who shall be citizens of India.