  We review the progress of research on intracluster planetary nebulae (IPN).
In the past five years, hundreds of IPN candidates have been detected in the
Virgo and Fornax galaxy clusters and searches are also underway in poorer
galaxy groups. From the observations to date, and applying the known properties
of extragalactic planetary nebulae, the intracluster light in Virgo and Fornax:
1) is significant, at least 20% of the total cluster stellar luminosity, 2) is
elongated in Virgo along our line of sight, and 3) may derive from
lower-luminosity galaxies, consistent with some models of intracluster star
production. A fraction of IPN candidates are not true IPN, but emission-line
sources of very large observed equivalent width (> 200 A). The most likely
source for these contaminating objects are Lyman-alpha galaxies at z ~ 3.1.
Follow-up spectroscopy of the IPN candidates will be crucial to discriminate
against high redshift galaxies and to derive the velocity field of the
intracluster stellar population.
