Simple Open EtherCAT Master (SOEM) is a lightweight, portable C library that implements the EtherCAT master protocol stack. It provides full support for CoE (CANopen over EtherCAT), FoE (File over EtherCAT), SoE (Servo over EtherCAT), EoE (Ethernet over EtherCAT), Distributed Clocks (DC), SII (Slave Information Interface), and network redundancy (dual NIC). The architecture is modular: an OS abstraction layer (osal) encapsulates platform-specific threading, timing, and memory operations, while a hardware abstraction layer (oshw) handles raw network access via sockets (Linux, Windows, RTEMS, VxWorks, etc.). The core library functions (ecx_*) operate on a context structure, enabling multiple master instances. Key implementation patterns include an indexed buffer management system for out-of-order frame handling, a thread-safe mailbox pool and queue, and a circular error ring buffer. The library is not a standalone application; it is designed to be embedded into industrial real-time control systems. Sample applications demonstrate cyclic process data exchange, SDO access, EEPROM manipulation, and firmware updates. The codebase is mature (2005–2025) and under active development, with a dual GPLv3/commercial license. Some technical drift exists due to bundled third-party WinPcap headers and several platform-specific contributor directories that may lag behind core updates. Overall, the library demonstrates strong engineering with clear separation of concerns and portability, making it a solid foundation for EtherCAT master development.