Isaac Newton held the view that light has corpuscular nature, even though he was the first one to use the superposition effects of light, known as Newton’s rings, which he used to accurately measure the curvature of his hand-polished plano-convex lens to construct his telescope. Huygens was Newton’s contemporary but steadfastly held the view that light is a wave phenomenon. This chapter explains the deeper physical significance of the Huygens–Fresnel’s (HF) diffraction integral, which remains as the mathematical workhorse for propagating light through free space, material media, and engineered boundary between media. The mathematical strength of HF integral lies with the fact that it obeys Helmholtz’s wave equation, which again obeys Maxwell’s wave equation. Almost all precision instrumental and measurement-oriented modeling of the propagation of EM waves, from radio frequencies to soft x-ray frequencies, are accurately carried out using the HF diffraction integral. The chapter considers evolutionary properties of diffraction patterns from near-field to far-field.