Interferometry is widely used for precision measurement of optical pathlengths and pathlength changes. Because optical pathlength can then be related to other parameters like refractive index, density, temperature, position, angle, and constituency, interferometry forms the basis of many types of sensors. In recent years such procedures have evolved to extremes, where extremely high precision measurements are required. Measurement have been reported with better than 1/10,000 wavelength resolutions, i.e. better than one angstrom. This allows the real time observation of the evolution of events that were not possible before, essentially on a molecular level. New applications include: the observation of the growth of a crystal with near single atomic layer sensitivity, the observation of minute changes in concentration and temperature of a solution, sub- microscopic changes in bubbles and particles in a solution, surface mapping to nearly atomic precision, observation of trace levels of gases, and others. New procedures that have been developed and the problems that must be overcome to push the precision of interferometric measurement to its limits will be reviewed and some recent applications of these methods will be described. These include phase shifting, heterodyne, resonance, and phase conjugate interferometry.