The main results presented in this review can be summarized as follows.
1) From a newly completed spectroscopic survey of nearby galaxies, it is confirmed that LINERs are extremely common, being present in about 1/3 of all galaxies with BT 12.5 mag. If all LINERs are regarded as active nuclei, they constitute > 70% of the AGN population, and AGNs altogether make up nearly half of all bright galaxies. These statistics should be regarded strictly as lower limits, because very faint AGNs can be hidden by brighter nuclear H II regions, while others deficient in ionized gas may be completely invisible.
2) Approximately half of all LINERs (the so-called transition objects) show evidence in their integrated spectra of contamination by circumnuclear star formation (H II regions). It is argued that the majority of transition objects are not powered exclusively by stellar photoionization.
3) AGNs (transition objects, LINERs, and Seyferts) preferentially occur in early-type galaxies, mostly of Hubble types E-Sbc. The presence of a bar has no visible effect on the probability of a galaxy hosting an AGN or on the level of activity of the AGN, when present.
4) LINERs share a number of similarities with Seyferts, but there are several subtle differences. The host galaxies of both classes of emission-line nuclei have nearly identical distributions of Hubble types, absolute magnitudes, and inclinations angles. The line luminosities and the general properties of the bulk velocity field of their NLRs are also comparable. However, the NLRs of LINERs differ from those of Seyferts in that the densities (in the low-density region) are lower, the reddenings are lower, the line widths are larger, and density stratification may be more common.
5) Based on the relative intensities of the narrow emission lines, at least 10% of all galaxies in the present survey are classified as Seyfert nuclei (types 1 and 2).
6) A BLR, as revealed by the presence of broad (FWHM 2000 km s-1) H emission, has been detected in approximately 20%-25% of all nearby AGNs, or in ~ 10% of all galaxies, implying that the space-density of broad-lined AGNs is much higher than previously believed. Some 25% of LINERs show broad H emission. If the ratio of LINERs with and without BLRs is assumed to be the same as the ratio of Seyfert 1s to Seyfert 2s (1:1.6), and if the low detection rate of broad H emission in transition objects can be attributed to observational selection effects, then at least 60% of all LINERs may be genuine AGNs.