If you are a Spring Boot developer, you must have come across the 'application.properties' or 'application.yml' file. Needless to say, these files reduces your development effort by minimizing the amount of XMLs that you were writing in a standard Spring based project. Moreover, we accommodate the common properties of our project in the form of key-value pairs in these files. Therefore, it becomes more than important to know all about these files. These are application properties file with the extension of either '.properties' or '.yml'. The topic of discussion for this article is 'How to Write Spring Boot Application Properties Files'. Furthermore, Spring Boot already provides some of the ready-made keys that we use most of the times in our project.