German citizenship may have been obtained through the following:
German citizenship acquired through birth in wedlock
Children born in wedlock between January 1, 1914, and December 31, 1974, acquired German citizenship only if the father was a German citizen at the time of their birth.
Children born to a German mother in wedlock between January 1, 1964 and December 31, 1974 only acquired German citizenship if they would have become stateless otherwise.
Children born in wedlock after January 1, 1975, acquired German citizenship if one of the parents was a German citizen at the time of their birth.
Children born in wedlock between April 1, 1953 and December 31, 1974, to a German mother and a non-German father did not become German citizens by birth. However, during the years 1975, 1976 and 1977, their parents could claim German citizenship for them. The deadline for this procedure irrevocably ended on December 31, 1977.
German citizenship acquired through birth out of wedlock
Children born out of wedlock to a German mother after January 1, 1914, acquired German citizenship. Children born out of wedlock to a German father after July 1, 1993 acquired German citizenship if (among other requirements) paternity had been established according to German law.
Children born out of wedlock to a German father before July 1, 1993 may acquire German citizenship by declaration before their 23rd birthday, if paternity has been established and if they have resided in Germany for at least three years. The declaration may only be made in Germany at the child's residence.
German citizenship acquired through adoption
If you were adopted as a minor by at least one German citizen on or after January 1, 1977, you are a German citizen. If the adoption occurred outside Germany, it must meet certain requirements.
Children who adopted by a German parent between January 1, 1959, and December 31, 1976, could have become a German citizen by declaration until December 31, 1977.
German citizenship acquired through legitimization
The marriage of parents of a child born out of wedlock was called “legitimization”. Children born out of wedlock between January 1, 1914 and June 30, 1998 could have acquired German citizenship through the marriage of their mother with their German father.
German citizenship acquired through marriage
Foreign women who married a German citizen between April 1, 1914 and March 31, 1953 acquired German citizenship automatically. Foreign women who married a German citizen between April 1, 1953 and December 31, 1969 could have acquired German citizenship under certain conditions, such as by declaration at time of marriage.
Since January 1, 1970 the acquisition of German citizenship through marriage does not exist anymore. A foreign spouse can only naturalize if certain requirements are met.
German citizenship acquired through other grounds
1. Children born in Germany after December 31, 1999, to foreign parents who were legal residents of Germany for at least eight years, acquire German citizenship too. However, between the age of 18 and 23 they must decide whether to keep German citizenship or the citizenship of their parents.
2. There are rare possibilities for naturalization, eg. for former German citizens.
Applicants must meet a host of requirements, including having to give up present citizenship(s) in order to become a German citizen, and being fluent in the German language.
For futher information regarding the naturalisation, please refer to the Staatsangehörigkeit