Naturalization may result in unintended consequences. Among those consequences is the possibility that a voluntary naturalization may result in the loss of a one's original citizenship.

Fortunately, under certain circumstances, a German citizen may avoid such a consequence by obtaining a Beibehaltungsgenehmigung (a "Retention Certificate"). More particularly, the Staatsangehörigkeitsgesetzt (the "StAG" or "German Nationality Act"), as amended, provides that Germans may apply for permission to retain their citizenship before being naturalized under the laws of another country. However, should a German fail to have a Retention Certificate in hand before obtaining another citizenship, then he or she will automatically lose German citizenship. *

Retention Certificates are issued by the Bundesverwaltungsamt (the "Federal Office of Administration") in Cologne, Germany. To obtain one, a German national must file an application at the appropriate German Consulate. The Consulate will preview the application before submitting it to the Federal Office of Administration for final approval; the latter's decision with regard to each application is discretionary.

In the Retention Certificate application, the applicant must show: (a) that he or she continues to have such close ties to Germany that keeping German citizenship upon acceptance of foreign citizenship is justified; and (b) why acquisition of foreign citizenship (in his or her particular situation) is advantageous or why it avoids significant disadvantages. Examples of acceptable specific benefits include training, study, employment, business relations or inheritance matters. In contrast, a general disadvantage, such as the lack of eligibility to vote, will not suffice. Moreover, the other state must allow dual citizenship.**

Of course, dual nationality can occur as the result of a variety of circumstances. For example, one may automatically acquire another citizenship through birth or marriage. In such cases, it is not likely that German citizenship will be lost. Rather, the loss of citizenship occurs only when a German citizen becomes a US citizen based upon application. Further informations are here.

* Since 2007, there are exceptions to this rule when applying for citizenship of other European Countries or Switzerland.

**The U.S. Government recognizes that dual nationality exists but does not encourage it as a matter of policy because of the problems it may cause. Claims of other countries on dual national U.S. nationals may conflict with U.S. law, and dual nationality may limit U.S. Government efforts to assist nationals abroad. The country where a dual national is located generally has a stronger claim to that person's allegiance.
See here.