Decisions of the Federal Constitutional Court are binding on the constitutional organs of the Federation and the Länder, as well as on all courts and public authorities (cf. § 31(1) of the Federal Constitutional Court Act). This binding effect is generally confined to the specific case in question.
Certain decisions of the Federal Constitutional Court, in particular decisions concerning the constitutionality of legislation, have the force of law (§ 31(2) of the Federal Constitutional Court Act); this means that they apply generally, and not just to the individual case. These decisions are published in the Federal Law Gazette.
If a law is found to be unconstitutional, the Federal Constitutional Court generally declares it void. When a law is declared void, this declaration also applies retroactively and has the same legal effect as if the law had never been enacted. In certain cases, the Federal Constitutional Court merely declares provisions to be incompatible with the Basic Law and determines a date after which they may no longer be applied. This particularly happens if the legislator has different options to remedy the violation of the Constitution, or if the disadvantages that result from the law immediately ceasing to have effect outweigh the disadvantages arising from its continued application on a transitional basis. The latter scenario often occurs in relation to tax laws, given that the statutory basis for levying a tax would otherwise be completely or partially set aside. In the transitional period, the legislator can be called upon to enact provisions that are compatible with constitutional law. In rare cases, the Federal Constitutional Court determines a transitional framework itself.
Beyond the case itself, declaring legislation void does not invalidate other judicial decisions based on that legislation (cf. § 79(2) of the Federal Constitutional Court Act). Non-appealable decisions remain valid, but can no longer be enforced. This does not apply to criminal proceedings, given the serious consequences they entail; if a criminal judgment is based on legislation that is void or incompatible with the Basic Law, the proceedings can be reopened even after the judgment has become final.