Duolingo ( ) is an American educational technology company which produces learning apps and provides language certification. On its main gamified language learning app, users can practice vocabulary, grammar, pronunciation and listening skills using spaced repetition. Duolingo offers over 100 total courses across over 40 distinct languages; including a small variety of constructed languages. The company uses a freemium model with over 500 million registered users. Duolingo offers a premium service which eliminates advertising and offers more features. Duolingo also offers the Duolingo English Test certification program and a literacy app for children called Duolingo ABC, and the company released an elementary level math app called Duolingo Math currently exclusive to iOS. History The idea for Duolingo was initiated at the end of 2009 in Pittsburgh by Carnegie Mellon University professor Luis von Ahn and his Swiss-born post-graduate student Severin Hacker. Von Ahn had sold his second company, reCAPTCHA, to Google and, with Hacker, wanted to work on something related to education. A driving motivation was Von Ahn's upbringing in Guatemala, where he saw how expensive it was for people in his community to learn English. Hacker (co-founder and current CTO of Duolingo) believed that "free education will really change the world" and wanted to supply people an outlet to do so. The project was originally sponsored by Luis von Ahn's MacArthur fellowship and a National Science Foundation grant. The founders considered creating Duolingo as a nonprofit organization but Von Ahn judged this model as unsustainable. An early revenue stream was as a crowdsourced translation service. This was replaced as a source of revenue by a Duolingo English Test certification program, advertising and subscription. In October 2011, Duolingo announced that it had raised $3.3 million from a Series A round of funding, led by Union Square Ventures, with participation from author Tim Ferriss and actor Ashton Kutcher's investing firm A-Grade Investments. Duolingo launched a private beta on November 27, 2011, and accumulated a waiting list of more than 300,000 people. The platform launched to the general public on June 19, 2012, at which point the waiting list had grown to around 500,000. In September 2012, Duolingo announced that it had raised a further $15 million from a Series B funding round led by New Enterprise Associates, with participation from Union Square Ventures. In November 2012, Duolingo released an iPhone app, followed an Android app in May 2013, at which time