An impression is how often your ad is shown. An impression is counted each time your ad is shown on a search result page or another site on the Google Network. Every time somebody sees your ad, that's an impression. You won't be charged by impressions, but by click. You can check how many impressions you have had and more data about the performance of your campaigns, on your Clever dashboard. If your campaigns just started running, then you should give them some time for the first results to appear. If after a few days, you still don't have any impressions you can contact your account manager and he will review your account for detecting the problem.

More info about impressions on Google

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