The coming of the laser made it necessary to understand coherent and partially coherent image formation. Examination of superposition theory shows that the intensity impulse response is basic to image formation analysis. Basic examples of coherent image formation are given. These principles are traced through the historical development of the hologram. A simplified approach is used to distinguish between and exemplify the underlying principles of: Fresnel, Fraunhofer, Sideband Fresnel, and Fourier Transform hologram.