Sometimes you have to jump in without all the facts. With the deadline to file a lawsuit bearing down on you, you may need to file that civil complaint before you are even sure who did what, when. You can often amend a civil complaint when new facts come in, sometimes with, sometimes without the judge's permission.
You file a legal complaint when you have something to complain about under the law, termed a cause of action. The paper you file with the court to start a civil lawsuit -- that is, a lawsuit that does not charge someone with a crime -- is called a complaint. In it, you describe who did what and the consequences to you. Since you must give a copy of the document to the people you are suing, they receive notice of your lawsuit and can file responses.
You can amend the who, what, where or when of your complaint by amending it. As long as new facts and claims relate to the same conduct or event that you described in the initial complaint, they are said to "relate back" to the initial filing date. This is important if the time you had to file a complaint passes. For example, if your complaint involves a car accident, your state law may give you one year from the accident date to sue. If you file the complaint on the final day of that year, new claims you raise in amendments are too late to be considered unless they relate back to the initial filing date.
Amending as of Right
The easiest way to amend your complaint is before the other side responds. Most states permit you to amend a complaint without court permission during this period. Some states require you to file an entire new complaint with the amendments included in it; others only ask for the amendments. The title of this document is "First Amended Complaint" and, like the original complaint, it must be given to the other parties.
Court Permission to Amend
If you want to amend the complaint after the other party filed a response, you need that party's permission to do so. If the other side doesn't agree, you must ask the court for permission to file an amended complaint, explaining when and how you learned the new facts and attaching a copy of the proposed amended complaint. Judges grant permission to amend when they decide that it is the fair thing to do. This depends, in part, on whether you could have discovered the "new" facts earlier with a little effort.