Abstract: Zero rating, which allows users to access select Internet services and content without incurring mobile data charges, is not a new concept. But it has become an object of debate as mobile carriers and major app providers have used it in the developing world to attract customers, with the goal of increasing Internet access and adoption. While some feel these programs violate net neutrality and create the potential for a two-tiered Internet, others argue that zero rating programs bring the developing world online and could be modified to uphold, rather than violate, net neutrality principles. At the same time, little research evaluating zero rating programs exists, and many different program formulations are lumped under the term “zero rating,” some of which are more compatible with net neutrality than others. In March of 2016, the Berkman Klein Center for Internet & Society gathered a diverse group of stakeholders from academia, the media, the government sector, industry, and the open software community to discuss the use of zero rating as a means to improve Internet adoption in the developing world and how and when it could be an effective tool, if at all. This paper captures the resulting dialogue and recommendations. The workshop summary is followed by a collection of briefing papers representing the viewpoints of many of the workshop participants. Key Findings: Many different models of industry initiatives currently fall into the loose definition of zero rating. Creating a better defined taxonomy of program parameters, technical mechanisms, and impacts may allow for greater nuance and understanding in the field, as well as more targeted regulatory responses. Universal Internet access and adoption is a common goal but one that requires significant investment in global infrastructure. Some assert that zero rating programs may serve as a helpful stopgap measures to increase access, while others argue that these programs contribute to the creation of a tiered Internet ecosystem without providing meaningful benefits to the targeted beneficiaries. Zero rating initiatives may be employed in pursuit of goals other than Internet adoption, such as an emergency services messaging system or security updates. The goals of a particular zero rating program may make it more or less controversial. More empirical research is required to fully assess the impact of specific zero rating initiatives, as well as zero rating generally, on Internet adoption in the developing world. This research will sometimes require access to usage information held by mobile carriers and zero rating service providers that should be handled with user privacy in mind.