This section describes the fundamental principles of optical waveguides. The primary design parameters for both step-index and graded-index waveguides are introduced, and the relationship between these parameters and key waveguide properties–such as the numerical aperture and the number of supported modes–is established. Coupling efficiency into waveguides is analyzed using the overlap integral. Finally, the bandwidth of the transferred optical signal is derived as a function of both material dispersion and the waveguide’s numerical aperture.