  We review the existing set of optical/UV/IR observations of Supernova 1993J,
concentrating heavily on optical data because these are by far the most
plentiful. Some results from theoretical modeling of the observations are also
discussed. SN 1993J has provided the best observational evidence for the
transformation of a SN from one spectral type to another, thereby providing a
link between Type II and Type Ib supernovae (SNe). This has strengthened the
argument that SNe Ib (and, by extension, SNe Ic) are core-collapse events. SN
1993J has remained relatively bright for 10 years; its late-time emission comes
from the collision of supernova ejecta with circumstellar gas that was released
by the progenitor prior to the explosion. The circumstellar material shows
strong evidence of CNO processing.
