Where do you place a retargeting pixel?

If you're looking to run a behavioral retargeting campaign, you should place the pixel just before the </body> tag on the pages that matter to you. In most cases, ecommerce sites will place the pixel across their entire site and try to bring back all possible customers. However, depending on your business, you may want to place the pixel on pages that show a strong intent to purchase, eg the top of your shopping cart funnel. This will lead to a smaller prospect list but most likely higher conversion ratios.

You can see some additional information in AdRoll's retargeting FAQs - http://www.adroll.com/help