The Basque Country, Euskadi, is a region in the north-east of Spain with a distinct language and culture. About two million people live there. There is also a smaller Basque population in France.
Under the dictatorship of General Franco, from 1939 to 1976, the Basque language and all Basque self-assertion were rigidly suppressed.
ETA, the group which was first accused of the Madrid bombings, but has denied them, was founded in 1959 to fight for Basque independence. In 1973 it killed Franco's prime minister, Luis Carrero Blanco.
Generally its attacks have been of a similar character - against targets taken as representing Spanish state oppression - rather than against the general population, though sizeable numbers of civilians have been killed by some of its attacks.
Since the overthrow of the dictatorship in 1976, the Basque Country has won a large degree of autonomy within the Spanish state, though still not self-determination.
The Basque Country is, on average, more prosperous than the rest of Spain, and many people have migrated there from the rest of Spain. 43% of the population report that their father was born somewhere else in Spain.
The large proportion of the population that consider themselves Spanish, or both Basque and Spanish, complicates the question of Basque nationalism.
Only 16% of the population have Basque as their first language. Only about a third read or speak it at all.
Thirty per cent of the population consider themselves Basque rather than Spanish, and another 17% more Basque than Spanish, which still leaves a majority considering themselves not predominantly Basque.
Twenty-eight per cent of the population say they favour Basque independence, though only 34% are definitely opposed to it.