In general, German citizenship is not established through birth on German territory but by descent from a German legal mother and/or a German legal father.
German citizenship may have been obtained through the following:
German citizenship by being born in wedlock
Children born in wedlock between Jan. 1, 1914 and Dec. 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 Jan. 1, 1964 and Dec. 31, 1974 only acquired German citizenship if they would have become stateless otherwise.
Children born in wedlock after Jan. 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 Dec. 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 Dec. 31, 1977.
German citizenship by being born out of wedlock
Children born out of wedlock to a German mother after Jan. 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 can only be made in Germany at the child's residence.
German citizenship by 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 happened outside Germany it has to meet certain requirements.
Children who had been adopted by a German parent between Jan. 1, 1959 and Dec. 31, 1976 could have become a German citizen by declaration until Dec. 31, 1977.
German citizenship by legitimization
The marriage of the parents of a child born out of wedlock was called “legitimization”. Children born out of wedlock between Jan. 1, 1914 and June 30, 1998 could have acquired German citizenship through the marriage of their parents.
German citizenship by marriage
Foreign women who married a German citizen between April 1, 1914 and Mar. 31, 1953 acquired German citizenship automatically.
Foreign women who married a German citizen between April 1, 1953 and Dec. 31, 1969 could have acquired German citizenship under certain conditions, esp. by declaration at time of marriage.
Since Jan. 1, 1970 the acquisition of German citizenship by marriage does not exist anymore. A foreign spouse can only naturalize if certain requirements are met.
German citizenship by other reasons
Children born in Germany after Dec. 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 will have to decide whether to keep German citizenship or the citizenship of their parents.
There are rare possibilities for naturalization especially for former German citizens.
Applicants have to meet a host of requirements; you typically have to give up your present citizenship(s) in order to become a German citizen, fluency in the German language is another precondition (please call the competent German Mission in the U.S. for further information).