This chapter explains the Real Time-Thread (RT-Thread) file system and introduces architecture, features, and usage of the RT-Thread virtual file system. A file system is a set of abstract data types that implements the storage, hierarchical organization, access, and retrieval of data. It is a mechanism for providing underlying data access to users. Device File System is a virtual file system component provided by RT-Thread. The Portable Operating System Interface (POSIX) standard is intended to achieve software portability at the source code level. In other words, a program written for a POSIX-compatible operating system should be able to compile and execute on any other POSIX operating system. RT-Thread supports the POSIX standard interface, so it is easy to port Linux/UNIX programs to the RT-Thread operating system. The device abstraction layer abstracts physical devices such as SD Card, SPI Flash, and Nand Flash into devices that are accessible to the file system.