  I review the basic properties of low-ionization nuclear emission-line regions
(LINERs), as well as the main ionization mechanisms thus far proposed for them.
There is substantial heterogeneity among LINERs, especially those with extended
kpc-scale emission. An extensive survey by Ho, Filippenko, & Sargent quantifies
with unprecedented accuracy the optical emission-line intensity ratios in the
nuclei of a complete sample of 486 nearby, bright galaxies. Over 40% of all
early-type galaxies in the sample can be classified as LINERs or LINER/H II
transition objects. A nonstellar continuum is detected in many LINERs,
suggesting that they are genuine AGNs powered by accretion onto a compact
object. Broad H alpha emission similar to that of Seyfert 1 nuclei is seen in
many LINERs. In some LINERs, the broad H alpha emission has two well-separated
peaks, probably indicative of an accretion disk.
