Phase referencing is the main observational tool that is used by radio
arrays in order to make high resolution images of extended radio
sources, or to determine the precise position of celestial objects,
natural and man-made. The fundamentals of phase referencing, examples
of results, second-order corrections, and the problems when the phase
stability is limited are discussed. Examples of projects that are in
progress or ready-to-go will be outlined. Special phase referencing
problems and possible solutions for high resolution ALMA observations
at high frequencies are also discussed.