This book discusses the principles and the latest progress of silicon optical modulators as cutting-edge integrated photonic devices on silicon-photonic platforms, which play key roles in modern optical communications with low power consumption, small footprints, and low manufacturing costs. Silicon Mach-Zehnder optical modulators are emphasized as the principal small-footprint optical modulator because of their superior performance in high-speed optical modulation at operational temperatures beyond 100 degrees Celsius without power-consuming thermo-electric cooling in spectral bands over 100 nm.