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Published online by Cambridge University Press: 04 August 2017
The various techniques for discovering active galaxies are reviewed, the main emphasis being on optical techniques and their application to emission-line galaxies with active nuclei. The three principal sets of selection criteria involve morphology, colours and emission lines. The major extant lists of emission line galaxies are summarized. Problems of detection are discussed, including selection effects and the use of automatic plate measuring machines. The difficulties of generating useful statistics on different classes of objects are highlighted, but no attempt is made in deriving new statistics or to summarize those in the literature.