Learn about the history of Kotlin adoption at Instagram: from shipping Buck support in 2019 to introducing the new Kotlin compiler from Jetbrains in 2023. Instagram has shifted an incredibly large codebase from Java to mostly Kotlin (Instagram surpassed 50% lines of code in Kotlin in early 2023). This presentation will cover some of the technical challenges and cultural challenges of moving a complex codebase from one language to another. We will cover Kosabi, a Meta-built source-only-ABI generation tool to improve Kotlin incremental build speeds. We will also touch on how Kotlin enables the future of UI development by enabling Jetpack Compose.