From the jetbrains site:<p><a href="http:&#x2F;&#x2F;blog.jetbrains.com&#x2F;kotlin&#x2F;2015&#x2F;11&#x2F;the-kotlin-language-1-0-beta-is-here&#x2F;" rel="nofollow">http:&#x2F;&#x2F;blog.jetbrains.com&#x2F;kotlin&#x2F;2015&#x2F;11&#x2F;the-kotlin-language...</a><p>JetBrains has been using Kotlin in production of IntelliJ IDEA, YouTrack and other products for quite a long time now. We have more than 250’000 LOC of Kotlin in production at the moment (plus about as much in the Kotlin project itself). While some of our projects are entirely written in Kotlin (account.jetbrains.com), others have introduced it to existing Java codebases, as we planned initially. We reached the level of interoperability where freely putting Kotlin alongside Java is transparent for Java clients: Java can be called from Kotlin and vice versa, sources can be mixed in one project, resulting .class files are totally compatible with Java tooling.