The militant group Al Shabaab (The Youth in Arabic), that has claimed responsibility for Thursday's deadly attack on a university in Kenya, has been battling the UN-backed government in Somalia, and has carried out a string of attacks in neighbouring Kenya.
The group, which is allied to the Al Qaeda, has been pushed out of most of the main towns in Somalia it once controlled, but it remains a potent threat.
It emerged as the radical youth wing of Somalia's now-defunct Union of Islamic Courts, which controlled Mogadishu in 2006, before being forced out by Ethiopian forces.
It is banned as a terrorist group by both the US and Britain and is believed to have between 7,000 and 9,000 militants.
Thursday's attack on the Moi University Garissa campus is the latest in a series of attacks it has carried out in neighbouring Kenya.
At least 17 people were killed more than 60 injured in the early morning attack while 535 still remained unaccounted for.
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Al Shabaab's biggest attack in Kenya was on Nairobi's Westgate shopping centre in 2013 when at least 68 people died.
Al Shabaab has also set up a recruiting network in Kenya, especially around the port city of Mombasa, which has a large Muslim population.
Although it has lost control of most towns and cities, it still dominates in many rural areas.
It was forced out of Somali capital Mogadishu in August 2011 and left the vital port of Kismayo in September 2012.
The loss of Kismayo has hit Al Shabaab's finances, as it used to earn money by taking a cut from the town's lucrative charcoal trade.
Al Shabaab advocates the Saudi-inspired Wahhabi version of Islam, while most Somalis are Sufis.
It has imposed a strict version of Islamic Sharia law in areas under its control, including stoning to death women accused of adultery and amputating the hands of thieves.
In a joint video released in February 2012, former Al Shabaab leader Ahmed Abdi Godane said he "pledged obedience" to Al Qaeda head Ayman al-Zawahiri.
The two groups have long worked together and foreigners are known to fight alongside Somali militants.