The short answer
An amended complaint replaces the original to fix, add, or drop claims or parties. Illinois courts allow amendments liberally. Relation back decides whether a late amendment is treated as filed on the original date for limitations purposes.
Where it fits in your case
Commonly after a motion to dismiss is granted without prejudice, or after discovery turns up new facts.
What to expect
Leave of court is generally needed once the defendant has responded (735 ILCS 5/2-616).
Relation back usually applies when the new claim arises from the same occurrence as the original.
New defendants face stricter requirements, including timely notice of the suit.
What we need from you
New facts or documents and the dates you learned of them.
Common misconceptions
“We can add anyone at any time.” Adding a party after the limitations period is risky.
“The old allegations still count.” An amended complaint generally replaces the original.