The on-line isotope mass separator ISOLDE is a facility dedicated to the production of a large variety of radioactive ion beams for many different experiments in the fields of nuclear and atomic physics, solid-state physics, materials science and life sciences. The facility is located at the Proton-Synchrotron Booster (PSB) at CERN, the European Organization for Nuclear Research. It is operated by the ISOLDE collaboration, whose present members are Belgium, Bulgaria, CERN, Denmark, Finland, France, Germany, Greece, Italy, Norway, Poland, Romania, Slovakia, South Africa, Spain, Sweden, Switzerland and the United Kingdom. The relation between CERN and the ISOLDE collaboration is specified in a Memorandum of Understanding from 2017 (following those in 1993, 2007 and 2012).