Kotlin's most valuable feature is arguably its 100% Java interop, which not only lowers the barrier to entry for its use in existing projects but gives it access to Java's rich ecosystem of libraries too. However, calling clean Java code from Kotlin, and calling clean Kotlin code from Java, is not always so clean. We will discuss strategies and tools that enable you to write code in either language that is clean, safe, and idiomatic whether it's being called from Java or from Kotlin. Kotlin has been embracing multiplatform as of late, as well; aside from targeting JS and Native, Kotlin allows targeting "common", where all of your code is written against a purely Kotlin stdlib, and can then be consumed by modules that target any platform. We will touch on the interop stories on these platforms, and how those of us who are likely using Kotlin on the JVM today can adapt and prepare our code to be idiomatic no matter where it may run in the future.