Phase diagram investigations in the quaternary Y-Ba-Cu-O system are critically reviewed. Especially that relations which are directly connected with technological applications are outlined in more detail. This topic comprises the univariant melt reactions at p(O₂) = 0.21 x 10⁵ Pa, the stability field of YBa₂Cu₃O_7-δ in dependence on oxygen partial pressure and temperature as well as oxygen content considerations of the melt. It is proved that the knowledge of phase relations is an useful basis for single crystal growth, preparation of textured samples, detection of impurities, estimation of the influence of containment materials.