Rule 1. Use an exclamation point to show emotion, emphasis, or surprise.
I'm truly shocked by your behavior!
Yay! We won!
Rule 2. An exclamation point replaces a period at the end of a sentence.
Incorrect: I'm truly shocked by your behavior!.
Rule 3. Do not use an exclamation point in formal business writing.
Rule 4. Overuse of exclamation points is a sign of undisciplined writing. Do not use even one of these marks unless you're convinced it is justified.