AdSense Limitations and Restrictions
Who qualifies for AdSense? Anyone who owns a website that doesn't violate the AdSense program policies can apply to become an AdSense publisher. Sites with adult material or that promote hatred or illegal activities are not acceptable, for example. See the AdSense Program Policies for more information.
Blog owners can also join the program, by the way, and display advertisements alongside their blog entries. This is easily done by inserting the AdSense code into the blog template. (AdSense for feeds is also of interest to bloggers.)
What if AdSense doesn't find any advertisements to display? This sometimes happens with obscure topics or those rarely searched for. If there are no advertisers bidding for keywords related to your content, there are simply no ads for Google to serve. By default, Google displays public service announcements (PSAs) in the space you provide for the ads, and you don't make any money when they're clicked. You can supply Google with a background color or the URL of an alternate advertisement to display if you'd rather not see any PSAs.