SN 1993J is to date the radio supernova whose evolution has been monitored in greatest detail and the one which holds best promise for a comprehensive theoretical-observational analysis .
The shell-like radio structure of SN 1993J has expanded in general accord with models of shock excited emission , showing almost circular symmetry for over 8 years , except for a bright feature at the south-eastern region of the shell that has been observed at every epoch .
The spectrum of SN1993J has flattened from \alpha \simeq - 1 to \alpha \simeq - 0.67 ( S _ { \nu } \propto \nu ^ { \alpha } ) .
The decelerated expansion can be modeled well with a single slope but apparently better with two slopes .
There are also intriguing hints of structure in the expansion curve .
The results by the two VLBI groups carrying out this research show general agreement , but also some differences .
A comparison of the optical and VLBI results about the details of the deceleration show some discrepancies .