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. 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. Kevin Most is a New York City-based software engineer who is passionate about Android, and excited about Kotlin. Most recently, he has been working at Foursquare on the City Guide Android app.