In 2011, Adblock Plus---the most widely-used ad blocking software---began to permit some advertisements as part of their Acceptable Ads program. Under this program, some ad networks and content providers pay to have their advertisements shown to users. Such practices have been controversial among both users and publishers. In a step towards informing the discussion about these practices, we present the first comprehensive study of the Acceptable Ads program. Specifically, we characterize which advertisements are allowed and how the whitelisting has changed since its introduction in 2011. We show that the list of filters used to whitelist acceptable advertisements has been updated on average every 1.5 days and grew from 9 filters in 2011 to over 5,900 in the Spring of 2015. More broadly, the current whitelist triggers filters on 59% of the top 5,000 websites. Our measurements also show that the program allows advertisements on 2.6 million parked domains. Lastly, we take the lessons learned from our analysis and suggest ways to improve the transparency of the whitelisting process.