Problems of detection, location, and identification of weak seismic events with a view toward a comprehensive test ban treaty have formed the main motivation for array deployment and associated research. In the past 15 yr, the large arrays LASA and NORSAR have been the main tools for this type of research. The event detection capability of arrays has proved superior to that of single stations, but event locations, while readily available, are seldom very accurate (error < 50 km) without supplementary information. The implied two-dimensional wave field sampling provided by arrays has been instrumental in understanding phenomena like the ambient seismic noise field, the extent of mantle heterogeneities, and their effect on short-period wave propagation. It is somewhat unfortunate that, due to limitations in handling the enormous amount of data involved, only a relatively small number of seismologists has had access to the high-quality array recordings; recent advances in computer technology might eliminate such problems in the near future. New technology also makes possible a new trend in array seismology, involving on the one hand world-wide deployment of small- and medium-sized arrays, and on the other hand opening up for applying array processing techniques to a global network of such stations. This paper gives an overview of past and present developments of array technology, and the associated seismological achievements. Emphasis is placed on the role of arrays in a potential comprehensive test ban treaty environment and on likely future developments in this particular field.