Once I've gotten the rehearsal invitation by word of mouth, the other two times it was by a formal invitation. The nice thing about a formal invitation is that you KNOW everyone received the invitation with all the information about when and where they need to be for both the reherasal and the dinner, and you don't get any unexpected guests to the rehearsal dinner. That being said, I've never received the rehearsal invitation with the wedding invitation. As a PP said, it's a separate event, and it's usually hosted by the groom's family. If you're the one hosting it then you can handle however you wish, but just know it doesn't need to be as formal as the wedding invite and *traditionally* is not included in the wedding invitation. However, I've never been a fan of the idea that just because something is usually done one way that means it always has to be that way. If you already know who is being invited to the rehearsal dinner, you're financing it (or the groom's family is in favor of having it included in the wedding invitation), and your wedding invitations are more informal/casual and less traditional anyway, then why not?