This chapter develops monochromatic scalar wave theory and Fourier optics for diffraction and imaging. Starting from the scalar wave equation, it introduces the Helmholtz equation, its Green’s function, and boundary value solutions via Green’s theorem. These tools are applied to diffraction by apertures using the Fresnel–Kirchhoff and Rayleigh–Sommerfeld formulations, clarifying Huygens’ principle and the role of boundary conditions. The chapter then introduces two-dimensional spatial Fourier transforms, shows how paraxial propagation in z is implemented in the spatial-frequency domain, and applies this to aperture fields so that far-field (Fraunhofer) and near-field (Fresnel) diffraction patterns emerge as Fourier transforms of the aperture function. Thin lenses are treated as phase shifters that implement Fourier transforms, leading to Fourier-plane imaging and a unified view of diffraction and imaging.