One of the most important applications of holographic interferometry is in the observation and analysis of the class of objects known as phase objects, which modulate the phase of a reflected or transmitted light wave and not the intensity. Phase objects include all transparent optical elements, and any volume in which the refractive index or optical path-length varies in the volume. This field of application has evolved significantly in recent years, partially owing to evolution of the general field of holography, and also aided by the developments in computer technology that simplify image analysis and laser technology that improves data recording quality and rates. The individual techniques that make possible the on-line recording and analysis of holographic interferograms are described. These include new recording materials and techniques such as thermoplastics and TV holography. The holography laser state of the art is reviewed. Advanced data reduction techniques including the most recent algorithms for fringe analysis and tomography are discussed. The high sensitivity techniques such as phase shift and heterodyne interferometry are presented. Finally, some recent applications are presented. This will include a discussion of a holographic system that was flown in the Spacelab 3 spaceflight and plans for its refinement and reflight in an upcoming mission.