RISC-V is an Instruction Set Architecture (ISA) that is both free and open. This means that the RISC-V ISA itself does not require a licensing fee, although individual implementations may do so. The RISC-V ISA is curated by a non-profit foundation with no commercial interest in products or services that use it, and it is possible for anyone to submit contributions to the RISC-V specifications. The RISC-V ISA is suitable for applications ranging from embedded microcontrollers to supercomputers. This book will first describe the 32-bit RISC-V ISA, including both the base instruction set as well as the majority of the currently-defined extensions. The book will then describe, in detail, an open-source implementation of the ISA that is intended for embedded control applications. This implementation includes the base instruction set as well as a number of standard extensions. After the description of the CPU design is complete the design is expanded to include memory and some simple I/O. The resulting microcontroller will then be implemented in an affordable FPGA development board (available from Elektor) along with a simple software application so that the reader can investigate the finished design.