The timing of the vote, the wording of the question, and allowing 16-17-year-olds to vote.
David Cameron hinted that the UK will soon join the US in air strikes on Islamic State.
As polls narrow, and a Yes vote in the Scottish referendum becomes increasingly possible, the PM insists he won't resign if the Union breaks up.
The Prime Minister and US President have vowed against isolationism in the face of Islamic militants and the situation in Ukraine; will Cameron finally clarify Britain's stance on air strikes against Islamic State?
Some in Labour are in danger of giving the appearance that they believe Cameron’s MPs will do a better job of finishing him off than they can.
After a video apparently showing the beheading of another American journalist has been released, a Cobra meeting will be held to discuss the threat to a UK hostage.
The decision of nine Conservative MPs elected in 2010 to stand down suggests many fear defeat - and makes it more likely.
The Prime Minister will make a statement in the Commons later today as parliament returns after recess and the coalition continues this morning negotiating how to widen anti-terror laws.
The PM declares that the "root cause" of terrorism is the "poisonous ideology of Islamist extremism".
Had the PM refused to promise a referendum, he would likely have been deposed as Conservative leader.