While writing code, we mostly care about that it should be efficient and should not break. How that code executes at runtime is a completely different story in case of Android. The .dex format has been here for years and everyone is accustomed to it. But there are a lot of complexities going on once the .dex reaches your device. This talk will focussed on analyzing on what happens after your code in Kotlin/Java is compiled, packaged and run on an actual device. We will take a deep dive into .dex format as well as see how runtime has evolved since Android 1.0 and taken a great leap with Android 5.0 and above. In the process, we will also learn how to write runtime efficient code.