In this paper I provide a brief introduction to astronomical interferometry at optical and infrared wavelengths. Two key concepts, central to understanding the basis and practice of interferometry are introduced: image formation with conventional telescopes, in particular the Fourier decomposition of images, and the nature and utility of measurements of the coherence function or mutual intensity. Thereafter I focus on optical/infrared interferometry, outlining how measurements of the coherence function are made at these wavelengths, how they can be used to interpret a source's structure, and what the principles of interferometric imaging tell us about the limitations expected for the current generation of arrays such as the VLTI.