Eta started out as a radical study group in 1959, but has been closely associated with violence for most of its history.
The organisation won many international admirers for its militant stand against Gen Franco's dictatorship, but became isolated when hardliners intensified a terrorist campaign during and after Spain's transition to democracy.
Influenced by the Irish peace process, Eta called a ceasefire in September 1998, but returned to terrorism in late 1999. In an apparent effort to imitate the IRA's Canary Wharf bombing, Eta attempted several "spectaculars" over the next two years, all aborted by the police.
But a bloody assassination campaign claimed the lives of 23 people in 2000, 15 in 2001, five in 2002, and three in 2003. Victims included councillors, academics and journalists as well as security forces personnel.
None of the group's attacks have proved fatal since May 30th, 2003, when Eta killed two policemen in the Navarran town of Sanguesa.
At first, this was not a change of policy, but a result of consistent and effective police operations in both France and Spain, which saw senior leaders and a number of active service units arrested.
Eta did continue to attempt assassinations, however. Two Basque police were almost killed in an ambush the following September, in which one of the terrorists died.
The train bombs on March 11th, 2004, in Madrid, which killed 191 people, were attributed to Eta, quite wrongly, by the conservative (PP) Spanish government of the day.
The bombs were the work of Islamists, and the government's apparent manipulation of information cost it the general election three days later, which brought José Luis Rodríguez Zapatero and the PSOE to power.
The scale of the Islamist bombings, and the revulsion they caused, forced a rethink about violence among Basque radicals, as did the IRA's moves towards decommissioning.
The police have found correspondence that suggests some leaders were still intent on killing: "You've done nothing; we need bodies on the table as soon as possible," one note read.
However, it seems that activists were reluctant to carry out killings when they thought their commanders might be negotiating with the government.
The last attack which was intended to cause death took place in January 2005.
Since then numerous small bombs have been exploded, at points throughout Spain, but it seems clear that casualties are not intended, and any injuries caused have been minor.
Batasuna leaders have described this situation as a "de facto truce", but any of these attacks could cause a death, however accidentally.
From Eta's point of view, the purpose of the current campaign (a bomb injured two people on Saturday night) is probably to say, "We haven't gone away, you know", and to downplay the impression that they might call a ceasefire from a position of military weakness, or even collapse.