Notifications stopped working for Messages (Mavericks) I'm using a 27" iMac with Mavericks. About 2 weeks ago, Messages stop creating notifications whenever I received a message from someone. I used to get a notification in the upper right corner of the screen if Messages was in the background, but now I don't get anything. As a result, when people send me IM messages while Messages is in the background, I don't see them until later when I see the Messages window.

I didn't change any settings or anything like that. They just stopped showing up.

I don't see anything in the Messages Preferences about turning Notifications on/off either.
The issue might be the way that you have your notifications for Messages set. If you go under Notifications -> Messages, you will see something that says "Messages alert style:" and will have the option to select None, Banners, or Alerts.

Make sure you don't have "None" set as your alert style. You can also set the badge counter to show for when you get a new message by clicking the "Badge app icon" option lower on the screen.