These are the main forces that may operate in the Mosul theatre, though not all will play a role in the fighting inside the city.
Prime Minister Haider al-Abadi announced early Monday that operations to retake Mosul had started.
These are the main forces that may operate in the Mosul theatre, though not all will play a role in the fighting inside the city:
Heavily-armed jihadists who have had years to prepare their defences in Mosul, which IS seized before sweeping through cities and towns to the south in 2014.
Iraqi forces have since regained significant ground, and Mosul is the last city the jihadists hold in the country.
The most elite forces in the country who have spearheaded most key battles against IS. But constant reliance on these troops over the past two years has taken a toll.
The Iraqi army has begun playing a more successful role in operations against the jihadists since it was revitalised by US-led training following several of its divisions collapsing during the IS offensive in the north two years ago.
Includes special forces units, paramilitary federal police and local policemen. Many Iraqi police forces have played roles more akin to those of soldiers in the war against IS.
A US-led international alliance is carrying out air strikes against IS in Iraq and Syria, and providing training, arms and equipment to forces opposing the jihadists.
There are more than 7,500 coalition military personnel deployed in Iraq, over half of them from the United States.
Most are in advisory or training roles, but special forces soldiers who have fought the jihadists on the ground have also been deployed and coalition forces near Mosul have also targeted IS with artillery.
Security forces of Iraq's autonomous Kurdish region who nominally answer to the federal government but in practice operate independently, battling IS along a long front in the country's north.
An umbrella organisation created in 2014, which includes a dizzying array of paramilitary forces who vary widely in skill and in the degree to which they are actually under government control.
The main groups in the Hashed al-Shaabi (Popular Mobilisation) are Iranian-backed Shiite militias including Ketaeb Hezbollah, Asaib Ahl al-Haq and Badr.
The Hashed has played a major role in stopping IS's advance as well as regaining ground from the jihadists, but forces within it have also carried out abuses including summary executions and kidnappings.
Iranian forces have provided advice and other assistance including funding for various militias fighting IS in Iraq.
Qassem Soleimani, the commander of Iran's Revolutionary Guards foreign operations wing, has been repeatedly pictured in Iraq during the war.
Deployed at a base near Mosul from which they have carried out artillery strikes against IS, Turkish troops are also present inside Iraq's autonomous Kurdish region.
The federal government has demanded their withdrawal, with the prime minister vowing that they will not take part in the operation to recapture Mosul, but Turkey has declined to do so.