The Federal Constitution of the Swiss Confederation (SR 10, German: Bundesverfassung der Schweizerischen Eidgenossenschaft (BV), French: Constitution fédérale de la Confédération suisse (Cst.), Italian: Costituzione federale della Confederazione Svizzera (Cost.), Romansh: Constituziun federala da la Confederaziun svizra (help·info)) of 18 April 1999 (SR 101) is the third and current federal constitution of Switzerland. It establishes the Swiss Confederation as a federal republic of 26 cantons (states). The document contains a catalogue of individual and popular rights (including the right to call for popular referenda on federal laws and constitutional amendments), delineates the responsibilities of the cantons and the Confederation and establishes the federal authorities of government.
The Constitution was adopted by popular vote on 18 April 1999. It replaced the prior federal constitution of 1874, which it was intended to bring up to date without changing it in substance.
Prior to 1798, the Swiss Confederacy was a confederation of independent states, not a federal state, and as such was based on treaties rather than a constitution. The Helvetic Republic of 1798–1803 had a constitution largely drawn up by Peter Ochs, in 1803 replaced by the Act of Mediation, which was in turn replaced by the Federal Treaty of 1815, which restored the Confederacy, while the individual cantons drew up cantonal constitutions, in most respects based on the Ancien Régime of the 18th century, but with notable liberal innovations in the constitutions of the new cantons of St. Gallen, Aargau, Thurgau, Ticino, Vaud and Geneva. The new cantonal constitutions in many cases served as precedents for the later federal constitution.
Following the French July Revolution in 1830, a number of large assemblies were held calling for new cantonal constitutions. The modifications to the cantonal constitutions made during this period of "Regeneration" remains the basis of the current-day cantonal constitutions. Vaud introduced the legislative popular initiative in 1846. Berne introduced the legislative optional referendum in the same year. 
The political crisis of the Regeneration period culminated in the Sonderbund War of November 1847. As a result of the Sonderbund War, Switzerland was transformed into a federal state, with a constitution promulgated on 12 September 1848. This constitution provided for the cantons' sovereignty, as long as this did not impinge on the Federal Constitution. The creation of a bicameral assembly was consciously inspired by the United States Constitution, the Federal Council and Council of States corresponding to the House of Representatives and Senate, respectively.
In a partial revision of 1891, the "right of initiative" was introduced, under which a certain number of voters could make a request to amend a constitutional article, or even to introduce a new article into the constitution. This mechanism is called federal popular initiative. Thus, partial revisions of the constitution could – from this time onward – be made at any time.