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Massive spectroscopic surveys like the SDSS have revolutionized the way westudy AGN and their relations to the galaxies they live in. A first step in anysuch study is to define samples of different types of AGN on the basis ofemission line ratios. This deceivingly simple step involves decisions on whichclassification scheme to use and data quality censorship. Galaxies with weakemission lines are often left aside or dealt with separately because one cannotfully classify them onto the standard Star-Forming, Seyfert of LINERcategories. This contribution summarizes alternative classification schemeswhich include this very numerous population. We then study how star-formationhistories and physical properties of the hosts vary from class to class, andpresent compelling evidence that the emission lines in the majority ofLINER-like systems in the SDSS are not powered by black-hole accretion. Thedata are fully consistent with them being galaxies whose old stars provide allthe ionizing power needed to explain their line ratios and luminosities. Suchretired galaxies deserve a place in the emission line taxonomy.

Emission line taxonomy and the nature of AGN-looking galaxies in the SDSS