The LGV Sud-Est (French: Ligne à Grande Vitesse Sud-Est; English: South East high-speed line) is a French high-speed rail line which connects the Paris and Lyon areas. It was France's first high-speed rail line, it has also been the most widely used line in France as well as being the busiest high-speed line in Europe.
Construction of LGV Sud-Est commenced in 1976, although development of the associated technologies had been underway for over a decade prior. Several key decisions, such as the use of overhead electrification instead of gas turbines, were influenced by geopolitical events as much as by innovations. On 22 September 1981, the inauguration of the first section between Saint-Florentin and Sathonay-Camp was attended by President François Mitterrand. It marked the beginning of the re-invigoration of French passenger rail service; ten million passengers travelled on the LGV Sud-Est within its first ten months of operation while domestic flights between Paris and Lyon declined substantially due to the effectiveness of TGV services.