The Basic Law for the Federal Republic of Germany (German: Grundgesetz fr die Bundesrepublik Deutschland) is the constitution of the Federal Republic of Germany.

The West German Constitution was approved in Bonn on 8 May 1949 and came into effect on 23 May after having been approved by the occupying western Allies of World War II on 12 May. It was termed "Basic Law" (German: Grundgesetz) to indicate that it was a provisional piece of legislation pending the reunification of Germany. However, when the latter took place in 1990, the Basic Law was retained as the definitive constitution of reunified Germany. Its original field of application (German: Geltungsbereich)that is, the states that were initially included in the Federal Republic of Germanyconsisted of the three Western Allies' zones of occupation, but at the insistence of the Western Allies, formally excluded West Berlin. In 1990, the Two Plus Four Agreement between the two parts of Germany and all four Allies stipulated the implementation of a number of amendments.

The German word Grundgesetz may be translated as either Basic Law or Fundamental Law. The term "constitution" (Verfassung) was avoided as the drafters regarded the Grundgesetz as an interim arrangement for a provisional West German state, expecting that an eventual reunified Germany would adopt a proper constitution, enacted under the provisions of Article 146 of the Basic Law, which stipulates that such a constitution must be "freely adopted by the German people". Nevertheless, although the amended Basic Law was approved by all four Allied Powers in 1990 (who thereby relinquished their reserved constitutional rights), it was never submitted to a popular vote, neither in 1949 nor in 1990. However, the Basic Law as passed in 1949 also contained Article 23 which provided for "other parts of Germany" to "join the area of applicability of the Basic Law" which was the provision that was used for German reunification from the constitutional standpoint. As the overwhelming consensus thereafter was that the German question was settled and to reaffirm the renunciation of any residual German claim to land east of Oder and Neie, the article was struck the same day as reunification came into force. An unrelated article on the relationship between Germany and the European Union was instead inserted in its place two years later.

In the preamble to the Basic Law, its adoption was declared as an action of the "German people", and Article 20 states "All state authority is derived from the people". These statements embody the constitutional principles that 'Germany' is identical with the German people, and that the German people act constitutionally as the primary institution of the German state. Where the Basic Law refers to the territory under the jurisdiction of this German state, it refers to it as the 'federal territory', so avoiding any inference of there being a constitutionally established 'German national territory'.

The authors of the Basic Law sought to ensure that a potential dictator would never again be able to come to power in the country. Although some of the Basic Law is based on the Weimar Republic's constitution, the first article is a protection of the human dignity ("Menschenwrde") and human rights; they are core values protected by the Basic Law. The principles of democracy, republicanism, social responsibility, federalism and rule of law are key components of the Basic Law (Article 20). Articles 1 and 20 are protected by the so called eternity clause ("Ewigkeitsklausel") Article 79 (3) that prohibits any sort of change or removal of the principles laid down in Articles 1 and 20.

Following the collapse and defeat of Nazi Germany in World War II, the victorious Allies asserted joint authority and sovereignty over Germany as a whole, collectively known as Allied-occupied Germany, defined as all territories of the former German Reich west of the Oder–Neisse line, having declared the destruction of Nazi Germany at the death of Adolf Hitler (the 1945 Berlin Declaration). The four powers divided "Germany as a whole" into four occupation zones for administrative purposes under the three Western Allies (the United States, the United Kingdom, France) and the Soviet Union, respectively. This division was ratified at the August 1945 Potsdam Conference. The four zones were agreed by the United States, United Kingdom and Soviet Union at the February 1945 Yalta Conference, setting aside an earlier division into three zones (excluding France) proposed by the September 1944 London Protocol.

All territories annexed by Germany before the war from Austria and Czechoslovakia were returned to these countries. The Memel Territory, annexed by Germany from Lithuania before the war, was annexed by the Soviet Union in 1945 and transferred to the Lithuanian SSR. All territories annexed by Germany during the war from Belgium, France, Italy, Luxembourg, Poland and Yugoslavia were returned to their respective countries.

Deviating from the occupation zones planned according to the London Protocol in 1944, at Potsdam, the United States, United Kingdom and the Soviet Union approved the detachment from Germany of the territories east of the Oder–Neisse line, with the exact line of the boundary to be determined in a final German peace treaty. This treaty was expected to confirm the shifting westward of Poland's borders, as the United Kingdom and United States committed themselves to support the permanent incorporation of eastern Germany into Poland and the Soviet Union. From March 1945 to July 1945, these former eastern territories of Germany had been administered under Soviet military occupation authorities, but following the Potsdam Conference they were handed over to Soviet and Polish civilian administrations and ceased to constitute part of Allied-occupied Germany.

In the closing weeks of fighting in Europe, United States forces had pushed beyond the agreed boundaries for the future zones of occupation, in some places by as much as 320 km (200 miles). The so-called line of contact between Soviet and U.S. forces at the end of hostilities, mostly lying eastward of the July 1945-established inner German border, was temporary. After two months in which they had held areas that had been assigned to the Soviet zone, U.S. forces withdrew in the first days of July 1945. Some have concluded that this was a crucial move that persuaded the Soviet Union to allow American, British and French forces into their designated sectors in Berlin, which occurred at roughly the same time, although the need for intelligence gathering (Operation Paperclip) may also have been a factor. In 1949, two German states of East and West Germany emerged.