Among all Real Madrid's rivals, Barcelona is the biggest. Their rivalry spans decades and is often regarded as one of the biggest and most fierce in the sport's history. The rivalry between the two Spanish giant clubs is known as "El Clsico" and is contested in domestic and foreign tournaments such as the UCL.
Being one of the three founding members of La Liga that have never been relegated from the top division since its inception in 1929 (along with Athletic Bilbao and Barcelona), Real Madrid holds many long-standing rivalries, most notably El Clsico with Barcelona and El Derbi Madrileo with Atltico Madrid.