This Field Guide provides a practical treatment of the fundamental theory of displacement measuring interferometry, with examples of interferometry systems and uses. It outlines alignment techniques for optical components, signal processing systems for phase measurements, and laser stabilisation for homodyne and heterodyne sources. The concept of displacement measurement uncertainty is discussed with a practical example of calculating uncertainty budgets. For practicing engineers, this Field Guide will serve as a refresher manual for error sources and uncertainty budgets. For researchers, it will bring new insight to the way in which this technology can be useful in their field. For new engineers, researchers, and students, it will also serve as an introduction into basic alignment techniques for breadboard-based optical systems.