  A rarity among supernova, SN 1993J in M81 can be studied with high spatial
resolution. Its radio power and distance permit VLBI observations to monitor
the expansion of its angular structure. This radio structure was previously
revealed to be shell-like and to be undergoing a self-similar expansion at a
constant rate. From VLBI observations at the wavelengths of 3.6 and 6 cm in the
period 6 to 42 months after explosion, we have discovered that the expansion is
decelerating. Our measurement of this deceleration yields estimates of the
density profiles of the supernova ejecta and circumstellar material in standard
supernova explosion models.
