The Kurdistan Workers' Party (PKK) has been a thorn in Turkey's side for decades.
The group, which has Marxist-Leninist roots, was formed in the late 1970s and launched an armed struggle against the Turkish government in 1984, calling for an independent Kurdish state within Turkey.
Since then, more than 37,000 people have died. During the conflict, which reached a peak in the mid-1990s, thousands of villages were destroyed in the largely Kurdish south-east and east of Turkey, and hundreds of thousands of Kurds fled to cities in other parts of the country.
In the 1990s, the organisation rolled back on its demands for an independent Kurdish state, calling instead for more autonomy for the Kurds.
In 1999, it suffered a major blow when its leader, Abdullah Ocalan, was arrested.
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