This part of ISO/TR 14999 gives terms, definitions and fundamental physical and technical relationships for interferometric measurements of optical wavefronts and surface form of optical elements. It explains why some principles of the construction and use of interferometers are important due to the wave nature of the wavefronts to be measured. Since all wavefronts with the exception of very extended plane waves do alter their shape when propagating, this part of ISO/TR 14999 also includes some basic information about wave propagation. In practice, interferometric measurements can be done and are done by use of various configurations; this part of ISO/TR 14999 outlines the basic configurations for two-beam interference. The mathematical formulation of optical waves by the concept of the complex amplitude as well as the basic equations of two-beam interference are established to explain the principles of deriving the phase information out of the measured intensity distribution, either in time or in space. Both random and systematic errors may affect the results of interferometric measurements and error types to be clearly differentiated are therefore described in this part of ISO/TR 14999.