The 21 cm HI line was first detected in 1951, and two years laterit was detected in an extragalactic system. Since then, HIobservations have made important contributions to the field of extragalactic Astronomy. Among others, this review of a 50–year history of the field touches upon highlights that refeer to: the understanding of the structure of spiral galaxies, mapping the distribution of dark matter on galactic and cosmic scales, elucidating the dynamics and evolution of groups and clusters of galaxies, the discovery of the topology of the large–scale structure of the Universe, the measurement of cosmological parameters and the investigation of structures at high redshift.