Duolingo

Duolingo, application known for its gamified approach to language learning, its wide accessibility, and its cast of quirky characters, which include Duo the owl. Duolingo is the most popular language-learning app worldwide, with more than 100 million monthly users. Its popularity can be attributed in part to its “freemium” model (having both a fully accessible free version and an optional paid version) and its engaging animations.

Background

Duolingo was created in 2011 by Guatemalan-American software developer Luis von Ahn, who was then a computer science professor at Carnegie Mellon University, and Swiss-born computer scientist Severin Hacker, one of von Ahn’s graduate students. Von Ahn had previously helped create CAPTCHA, whose acronym is short for “completely automated public Turing test to tell computers and humans apart.” It verifies whether users are humans or bots by having users decipher distorted characters that bots cannot decipher. His enhanced CAPTCHA technology, reCAPTCHA, which identifies more-advanced bots, was bought by Google in 2009.

In 2024 von Ahn explained that he and Hacker had created Duolingo because, “in non-English-speaking countries, English is pretty transformational.” Thus, they created an application that could help users learn English. The idea was then expanded to languages other than English. As of 2025 Duolingo has more than 40 languages available. The most popular languages, aside from English, are Spanish, French, and German, but the app also features constructed languages, such as High Valyrian (from the popular HBO show Game of Thrones) and Klingon (from the science-fiction franchise Star Trek).

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After starting Duolingo as a website, von Ahn and Hacker made a smartphone app. Many other companies at the time had so-called mobile “companion apps” with some functionality, but the Duolingo app had all the website’s features and became the company’s main platform.

Design and features

Duolingo’s interface has gamelike features, inviting users to complete sequences consisting of one- to four-minute lessons. These lessons make up broader units focused on certain vocabulary or grammatical concepts. If users answer questions incorrectly, they may lose a “heart” (a graphic inspired by the video game The Legend of Zelda). However, the heart is simple enough to earn back. Users earn “experience points,” or XP, when they complete lessons. Duolingo also has a social aspect, allowing users to send one another XP “boosts,” which help users earn more points faster. Courses are created by “language learning experts” who have backgrounds such as linguistics and teaching. They are based on the common European framework of reference for languages (CEFR). The system is useful for assessing what users should be proficient at during specific points in their language-learning journey.

Duolingo’s most iconic feature is its bright green owl mascot, aptly named Duo, who attempts to make users feel guilty unless they continue their language lessons day after day. Duolingo’s application features Duo’s face, which often changes to reflect the character’s mood. At times, the cartoon owl has been designed to look sickly, which Duolingo’s design team attributes to his tiredness from attempting to persuade users to return to their lessons. Duo has prompted many online memes in which users claim that Duo has threatened them with violence if they do not continue their lessons. Duolingo’s team has played into this reputation by devising harmless pranks and social media posts. In February 2025 the owl faked his own death, and the company sassily announced that he had “died waiting for you to do your lesson, but what do we know.”

Duolingo’s approach to notifications has been successful at retaining users. Streaks, in which the app tracks how many consecutive days users have done their lessons, are meant to keep users coming back day after day to keep their run of progress active. If users miss a day, the streak is broken (that is, unless they have implemented a limited-use “streak freeze”). Duolingo uses an artificial-intelligence-based bandit algorithm (an algorithm that helps to optimize decision-making) to send reminder notifications. The algorithm works by predicting which notifications users will be most likely to click on, given their prior behavior.

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Efficacy

Late in the first decade of the 2000s the field of mobile-assisted language learning grew significantly, mainly because smartphones became more popular and changed how users approached language learning. Some studies have shown that learning languages through Duolingo is comparable to classroom learning.

In 2017 a study of third- and fourth-grade students that compared Spanish skills learned via Duolingo with those learned in the classroom found no significant difference between the two methods. Similar results were reported in a study in 2019 that involved college students who learned Italian on Duolingo versus those who learned by using an online slideshow. In 2024 a study of native Japanese, Spanish, and Portuguese speakers who were learning English revealed notable skill improvements following Duolingo English courses.

However, such studies have faced criticism for their small sample sizes and lack of standardization. For example, in 2021 a review of Duolingo studies found that many had focused solely on English as the studied language and had analyzed Duolingo’s design and features rather than examining the learning trends associated with such features. Moreover, many studies had not accounted for learners’ motivations for learning a language or for their individual backgrounds.