  Over a decade ago, a group of supernova explosions with peak luminosities far
exceeding (often by >100) those of normal events, has been identified. These
superluminous supernovae (SLSNe) have been a focus of intensive study. I review
the accumulated observations and discuss the implications for the physics of
these extreme explosions. SLSNe can be classified into hydrogen poor (SLSNe-I)
and hydrogen rich (SLSNe-II) events. Combining photometric and spectroscopic
analysis of samples of nearby SLSNe-I and lower-luminosity events, a threshold
of M_g<-19.8 mag at peak appears to separate SLSNe-I from the normal
population. SLSN-I light curves can be quite complex, presenting both early
bumps and late post-peak undulations. SLSNe-I spectroscopically evolve from an
early hot photospheric phase with a blue continuum and weak absorption lines,
through a cool photospheric phase resembling spectra of SNe Ic, and into the
late nebular phase. SLSNe-II are not nearly as well studied, lacking
information based on large sample studies. Proposed models for the SLSN power
source are challenged to explain all the observations. SLSNe arise from massive
progenitors, with some events associated with very massive stars (M>40 solar).
Host galaxies of SLSNe in the nearby universe tend to have low mass and
sub-solar metallicity. SLSNe are rare, with rates <100 times lower than
ordinary SNe. SLSN cosmology and their use as beacons to study the
high-redshift universe offer exciting future prospects.
