Chemical weapons are gases or other chemicals designed to strike down large numbers of people over a wide area.
They were first developed and used in the First World War in 1915, when poison gas was used on soldiers.
The weapons were so devastating that, for many, the idea of using them again became unacceptable and they are now banned under international law.
188 nations around the world have signed up to the chemical weapons convention, which bans their use and production. Seven countries didn't sign up - one of them was Syria.
Have they been used in Syria?
There have been claims of chemical weapons being used by both sides in Syria.
Until now, powerful western countries like America have stayed out of the fighting in Syria but if chemicals have been used, it could force them to take action.
The United Nations have asked for their inspectors to be allowed in to confirm whether chemical weapons have been used.
If they have, the effect it could have on the war tearing Syria apart could be massive.