As part of an optical spectroscopic survey of nearby , bright galaxies , we have identified a sample of over 200 emission-line nuclei having optical spectra resembling those of giant extragalactic \hii regions .
Such “ \hii nuclei , ” powered by young , massive stars , are found in a substantial fraction of nearby galaxies , especially those of late Hubble type .
This paper summarizes the observational characteristics of \hii nuclei , contrasts the variation of their properties with Hubble type , and compares the nuclear \hii regions with those found in galaxy disks .
Similarities and differences between \hii nuclei and luminous starburst nuclei are additionally noted .

Nuclei in early-type spirals ( S0–Sbc ) on average have low excitation , and hence high oxygen abundance ( from \sim 1.1 to 3.3 times the solar value ) , whereas those in late-type systems ( Sc–I0 ) have excitations spanning a wide range ( corresponding to less than 0.25 to 3.5 times the solar oxygen abundance ) .
The H \al luminosities of early-type nuclei greatly exceed those of later types .
The enhancement of massive star formation may be linked to the higher efficiency with which bars can drive gaseous inflow in systems with prominent bulges .
The early-type systems also have higher amounts of internal extinction and higher electron densities .

The physical properties of \hii nuclei resemble those of giant \hii regions in spiral disks in some ways , but differ in several others .
The two groups emit comparable H \al luminosities and generally have similar electron densities .
Because of their unique location in the galaxies , nuclear \hii regions are characterized by much higher oxygen abundances . \hii nuclei systematically emit stronger low-ionization forbidden lines than disk \hii regions , confirming a trend recognized by Kennicutt , Keel , and Blaha .
We discuss several possibilities for the origin of the spectral variations .