This chapter presents the fundamental computational methods used to construct numerical models of coherent optical systems. Such systems include lasers, Shack–Hartmann wavefront sensors, interferometers, and general systems using coherent optics. The core techniques rely on calculating the diffraction integral numerically using Fast Fourier transform. A spectral propagation method is derived by decomposing the wave field into a basis of plane waves. Propagation of light field via numerical solution of scalar parabolic equation is introduced as an alternative to integral-based methods. This approach offers the significant advantage of naturally incorporating the parameters of non-uniformly absorbing and refracting media. To achieve this, a parabolic equation is derived using the slowly varying amplitude approximation. Examples implemented in MATLAB are provided to illustrate the propagation and focusing of light waves. Finally, coordinate transformations are introduced as a tool to simplify the analysis of divergent and convergent beams, extending the applicability of the methods to a wider range of optical configurations.