This book covers the Cortex-M, a 32-bit MCU (microcontroller unit) built with an ARM processor core, and the Mbed OS, an operating system developed to efficiently manage processors. The book is largely divided into five parts. In Part 1, the background of the microcontroller, necessity, characteristics, and configuration of the Mbed OS will be described. Part 2 is about programming for basic input/output devices, and lays the foundation by learning not only basic functions but also their utilization. In studying basic input/output functions supported by Mbed OS over several chapters, it is configured to first look at basic concepts and develop utilization skills through practice using those functions. For example, learning the functions of the Timer class will help you to think from various viewpoints about the structure of the program. In Part 3, the major communication methods such as UART, I2C and SPI necessary to design and realize an embedded system will be studied since they have not been covered in detail in despite of their importance. In addition to the interface with peripherals using these communication methods, topics about efficient communication using callback functions are also examined. Part 4 covers advanced programming topics related to Bus I/O, RTOS, and Circular Buffer. In particular, RTOS classes such as Thread, Mutex, and Queue will be learned through various examples. Part 5 introduces projects that require multiple functions and concepts of Mbed OS, so that readers can improve their application skills. For example, we will challenge to develop ultrasonic rangefinder, stepper motor drive, encoder reading, DC motor PID control, Lidar scanner, and AHRS (attitude heading reference system) using IMU (inertial measurement unit) sensor to enhance the overall application capabilities and further to obtain practical system configuration skills.