The short answer
The complaint is the document that starts a lawsuit. It names the parties, states the facts, lists each legal claim (called a count), and says what the plaintiff wants. Illinois requires specific facts, not just conclusions.
Where it fits in your case
It is filed at the very start. The defendant’s response deadline runs from service of the complaint, not from the filing date.
What to expect
Fact pleading. Illinois complaints must allege facts supporting each element of each claim. Federal complaints must state a plausible claim.
Exhibits. A claim based on a written contract generally must attach the contract (735 ILCS 5/2-606).
Information and belief. Facts the plaintiff cannot yet confirm may be alleged on information and belief if there is a good-faith basis.
What we need from you
If you are filing: the contract, key correspondence, a timeline, and your damages figures. If you were sued: the complaint and the date and manner you received it.
Common misconceptions
“The complaint is the whole story.” It is one side’s allegations, not findings.
“We can fix mistakes later.” Amendments are usually allowed, but statements in a complaint can be used against the party that made them.