This opening chapter provides the fundamentals of some principal photonic devices that are essential for processing of ultrafast electrical and electronic signals in the photonic domain, including optical communications and microwave photonics. It describes the basic structures, the benefits, and the needs of photonic signal processing for the advancement of such systems. The device structures, fabrication, and characteristics of operating functions of the optical amplifiers, optical filters, and optical modulators are examined. The transmittance of photonic components can be represented, in Chapter 2, in the form of signal flow and blocks of photonic signal subsystems.