From what I understand, it is allowed, but I wanted to make sure. I have decided that less is more and I am in the process of deleting pages containing outdated information. Instead of re-directing the pages I have set up a custom error page telling visitors where to go to find their way around with a link provided. My question is, can I put Adsense on the custom 404 error page? Maybe my visitor doesn't feel like "finding their way around" and would prefer to move on. I'd prefer they click an ad instead of clicking that "back" button.