In this dissertation, I present the RISC-V instruction set architecture. RISC-V is a free and open ISA that, with three decades of hindsight, builds and improves upon the original Reduced Instruction Set Computer (RISC) architectures. It is structured as a small base ISA with a variety of optional extensions. The base ISA is very simple, making RISC-V suitable for research and education, but complete enough to be a suitable ISA for inexpensive, low- power embedded devices. The optional extensions form a more powerful ISA for general- purpose and high-performance computing. I also present and evaluate a new RISC-V ISA extension for reduced code size, which makes RISC-V more compact than all popular 64-bit ISAs.