Duolingo is the most downloaded and top grossing education app on iOS and Android, with over 300 million users. It's the most popular way to learn a language online. Every day, it makes choices between making a ton of money vs. keeping learning content free, making the app easier to drive up engagement vs. really teaching a language well, gamifying things and making the app more addictive vs. ensuring it's a healthy app for our users to use. In this talk, Karin Tsai will explain how Duolingo not only evolved processes to keep its mission and culture while still continuing to grow and produce great metrics wins, but discovered that it was a good business decision to do so. Attendees will get insights from seven years' worth of A/B tests of surprising findings, product insights, and difficult decisions.