I recently had a site targeted campaign that was doing pretty well...I changed the wording of the ad and all of a sudden I got 0 clicks and 0 impressions. How is it possible that not a soul saw my ad? As I could not continue getting 0 results, I changed the ad back to what it said before...and it is still getting 0 clicks and 0 impressions. Am I doing something wrong?

The key thing to know is that all new or edited ads must be reviewed and approved before they can appear on sites in the Google Network. So, for new (or edited) ads which are shown only on the Google Network, including image ads and site-targeted ads, you won't receive any impressions until they have been reviewed and approved.

On a related subject, in some cases it's possible for new or edited keyword targeted ads to appear on Google before review -- but they will still need to be reviewed and approved before being shown on Google Network sites. In such cases, an advertiser who is opted-in to the Google Network may notice an increase in impressions once their ads have been reviewed and approved -- since at this point the ads have the potential to be shown on many sites in addition to Google.

Tip: When your advertising needs allow it, you may wish to submit your new or edited ads early to mid-week. Here's why: ads are reviewed on a more limited basis over the weekend, so ads submitted late Friday or over the weekend may take longer to review than those submitted earlier in the week. Also, be aware that review time may vary with the volume of ads being edited and created -- so ads tied to a major retail holiday or event ideally would be submitted a bit in advance to account for higher volumes.

Posted by Blake, Inside AdWords crew