Twitter is a social media web site created in 2006 that allows users to post Tweets, which are text-based messages containing up to 140 characters. It has grown exponentially in popularity; now more than 340 million Tweets are sent daily, and there are more than 140 million users. Twitter has become an important tool in medicine in a variety of contexts, allowing medical journals to engage their audiences, conference attendees to interact with one another in real time, and physicians to have the opportunity to interact with politicians, organizations, and the media in a manner that can be freely observed. There are also tremendous research opportunities since Twitter contains a database of public opinion that can be mined by keywords and hashtags. This article serves as an introduction to Twitter and surveys the peer-reviewed literature concerning its various uses and original studies. Opportunities for use in ophthalmology are outlined, and a recommended list of ophthalmology feeds on Twitter is presented. Overall, Twitter is an underutilized resource in ophthalmology and has the potential to enhance professional collegiality, advocacy, and scientific research.