Government forces have been working their way north in recent months, notching up key victories against the jihadists, who seized Tikrit last June.
But the overwhelmingly Sunni Arab city on the Tigris river has resisted them several times and is their toughest target yet.
Situated 160 kilometres (100 miles) north of Baghdad, the city is the capital of Salaheddin province.
The province is named for the Muslim warlord of the same name who seized Jerusalem in 1187 and whose birthplace was Tikrit.
Saddam modelled himself on Salaheddin, a Kurd considered to the one of Islam's most brilliant strategists, a reputation he gained during his fight against the Christian crusaders.
An official announcement of the fall of Tikrit signalled the symbolic end of Saddam's regime in the US-led invasion of 2003. The city was the last major town be captured by coalition forces.
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Saddam's legacy persists, with remnants of his Baath party having collaborated with IS in attempting to topple the Shiite-dominated government.
The city benefited from its loyalty to Saddam and had some of the best hospitals, schools, and roads in Iraq, as well as a university.
It also has several presidential palaces and other official buildings.
Not far away is Awja, the village where Saddam was born on April 28, 1936.
It is the site of a richly-decorated mausoleum for Hussein al-Majid, Saddam's father, but under the previous regime Iraqis were not allowed to go there.
During Saddam's time the entrance to the city was adorned with a gigantic arch, showing Saddam, gun in hand, and Salaheddin, a sword in the air.